This is a workaround for a bug that showed up after gitlab migration. See issue
infrastructure/issues0#62 for further
discussion.

Merge branch 'fix-eigenvectors-some-more' into 'master'

ref:core/dune-common\> This merge request fixes three bugs in the
eigenvalue/eigenvector code for 2x2 and 3x3 symmetric matrices. Surprisingly,
all three bugs can be triggered by using particular diagonal matrices.

See merge request [core/dune-common!839]

  [core/dune-common!839]: gitlab.dune-project.org/core/dune-common/merge_requests/839

Add a test for the 2x2 zero matrix as well, even though that
appears to work properly.

The code for eigenvalues of diagonal 3x3 matrices didn't take
the matrix scaling properly into account.

Surprising how difficult it is to compute eigenvalues of
diagonal matrices...

The eigenvector code used to fail for certain seemingly simple
diagonal 2x2 matrices.  The reason was that the code tried
to avoid certain all-zero matrix columns by testing whether
the norm is (close to) zero.  That proved to be surprisingly
unstable.  The fix here is simple: Pick the column with the
larger column norm (we know they are not both zero).

Also add a test case to make sure the bug is gone.

Merge branch 'feature/find_ptscotch' into 'master'

ref:core/dune-common\>

### Summary

This is a small improvement of the existing FindPTScotch cmake module. It
allows to search only for Scotch or PTScotch and thus it is possible to use
Scotch without PTScotch or if no MPI is available.

See merge request [core/dune-common!831]

  [core/dune-common!831]: gitlab.dune-project.org/core/dune-common/merge_requests/831

Merge branch 'eigenvalues-of-singular-matrices' into 'master'

ref:core/dune-common\> The eigenvector algorithm for symmetric FieldMatrices
was buggy, it does not handle singular matrices well. Add a test and fix the
bug.

See merge request [core/dune-common!836]

  [core/dune-common!836]: gitlab.dune-project.org/core/dune-common/merge_requests/836

Plus, add a test that eigenvalues and eigenvectors are not NaN.
The previous test would not actually catch this, because
"NaN > some_number" is always false.

The eigenvectors were correct, but their ordering didn't match
the one of the eigenvalues.

And document the algorithm while we are there.

Merge branch 'feature/sparseRange' into 'master'

ref:core/dune-common\> This helper allows to use for-loops with structured
bindings for sparse ranges R whose iterators it provide an it->index() method,
like this

    for(auto&& [A_i, i] : sparseRange(R))
      doSomethingWithValueAndIndex(A_i, i);

This also adds a test and fixes the placement of the range-utilities in the
appropriate doxygen group to make them easier to discover.

See merge request [core/dune-common!834]

  [core/dune-common!834]: gitlab.dune-project.org/core/dune-common/merge_requests/834

This helper allows to use `for`-loops with
structured bindings for sparse ranges `R`
whose iterators `it` provide an
`it->index()` method, like this

```cpp
for(auto&& [A_i, i] : sparseRange(R))
  doSomethingWithValueAndIndex(A_i, i);
```

Merge branch 'feature/extend-transformedrangeview' into 'master'

ref:core/dune-common\> This extends TransformedRangeView by also allowing to
apply a transformation to the iterator directly. While the (existing) default
mechanism is to use f(*it) where f is the transformation and it an iterator of
the wrapped range, the new iterator based transformation uses f(it) instead.
This allows to incorporate additional information from the iterator in the
transformation.

Using the new functionality one can e.g. easily implement structured bindings
support for sparse ranges using

    template<class Range>
    auto sparseRange(Range&& range) {
      return Dune::transformedRangeView(std::forward<Range>(range), Dune::taggedCallback<Dune::IteratorTransformationTag>([](auto&& it) {
          return std::tuple<decltype(*it), decltype(it.index())>(*it, it.index());
      }));
    }

    ...

    for(auto&& [entry,col] :  sparseRange(matrix[row]))
      y[row] += entry*x[col];

Some more implementation details:

-   This adds the TaggedCallback<Tag,F> helper class and the corresponding
    generator function taggedCallback<Tag>(f) for tagging a callback. Using
    this mechanism we can provide additional information on the callback.
-   If the TransformedRangeView is provided with a callback of type
    TaggedCallback<IteratorTransformation,F>, then it iterator based
    transformation is used.

See merge request [core/dune-common!781]

  [core/dune-common!781]: gitlab.dune-project.org/core/dune-common/merge_requests/781

Merge branch 'feature/python-force-library-so-suffix-for-macos' into 'master'

ref:core/dune-common\> The issue is discussed in this old thread on
stackoverflow:
[https://stackoverflow.com/questions/2488016/how-to-make-python-load-dylib-on-osx]

Somehow Python refuses to import dynamic libraries with the suffix .dylib (I
get an import error) which is the common suffix for shared libs in macOS.
Telling CMake to use .so instead as suggested in the stackoverflow thread
works for me.

The Python libs on for my Python 3.7 from homebrew all have .so endings just
like in the stackoverflow answer.

Fixes [#201]

See merge request [core/dune-common!799]

  [https://stackoverflow.com/questions/2488016/how-to-make-python-load-dylib-on-osx]:
    https://stackoverflow.com/questions/2488016/how-to-make-python-load-dylib-on-osx
  [#201]: gitlab.dune-project.org/NoneNone/issues/201
  [core/dune-common!799]: gitlab.dune-project.org/core/dune-common/merge_requests/799


Closes #201

Merge branch 'issue/add_suitesparse_flags' into 'master'

ref:core/dune-common\>

### Summary

Quickfix of AddXXXFlags files with target_compile_definitions

See merge request [core/dune-common!833]

  [core/dune-common!833]: gitlab.dune-project.org/core/dune-common/merge_requests/833

Merge branch 'feature/ptscotch_test' into 'master'

ref:core/dune-common\>

### Summary

This MR adds a test scotchtest.cc to check that scotch is found and works
correctly. The actual scotch test just runs a few standard functions to create
a graph and check it validity. The test is added to test the cmake
find-package FindPTScotch.cmake and its add_dune_ptscotch_flags macro.

See merge request [core/dune-common!829]

  [core/dune-common!829]: gitlab.dune-project.org/core/dune-common/merge_requests/829

Merge branch 'bugfix/issueWithMultipleTypesInGenerator' into 'master'

See merge request [core/dune-common!832]

  [core/dune-common!832]: Nonecore/dune-common/merge_requests/832

Merge branch 'feature/find_ptscotch' into 'master'

ref:core/dune-common\>

### Summary

Quick-Fix of a typo in the cmake variable name

See merge request [core/dune-common!830]

  [core/dune-common!830]: gitlab.dune-project.org/core/dune-common/merge_requests/830

Merge branch 'feature/find_ptscotch' into 'master'

ref:core/dune-common\>

### Summary

This MR adds imported-targets to the FindPTScotch.cmake module, following
modern cmake guidelines.

### Details

Instead of exporting include directories and libraries as variables, the find
modules exports an imported target that contains all the necessary information
to include and link PTScotch. The find module introduces two imported targets:
PTScotch::Scotch for the sequential graph partitioning library and
PTScotch::PTScotch for the parallel library that additionally depends on MPI.

### ToDo

-   [x] Add version detection for (PT)Scotch library

See merge request [core/dune-common!820]

  [core/dune-common!820]: gitlab.dune-project.org/core/dune-common/merge_requests/820

Merge branch 'feature/find_suitesparse' into 'master'

ref:core/dune-common\>

### Summary

This MR adds imported-targets to the FindSuiteSparse.cmake module, following
modern cmake guidelines.

### Details

Instead of exporting include directories and libraries as variables, the find
modules exports an imported target that contains all the necessary information
to include and link SuiteSparse libraries. The module allows to search for
various SuiteSparse components, like UMFPACK or SPQR, and introduces an
imported target for each of them in the form: SuiteSparse::<component>.
Additionally the combined imported target SuiteSparse::SuiteSparse is defined
to link against all found components.

Difference to old FindSuiteSparse module: It searches for all possible
components and creates corresponding component targets. The collective target
SuiteSparse::SuiteSparse then only contains the requested components.

See merge request [core/dune-common!823]

  [core/dune-common!823]: gitlab.dune-project.org/core/dune-common/merge_requests/823

This extends `TransformedRangeView` by also allowing
to apply a transformation to the iterator directly.
While the (existing) default mechanism is to use `f(*it)`
where `f` is the transformation and `it` an
iterator of the wrapped range, the new iterator
based transformation uses `f(it)` instead. This
allows to incorporate additional information
from the iterator in the transformation.

Using the new functionality one can e.g. easily
implement structured bindings support for sparse
ranges using

```cpp
template<class Range>
auto sparseRange(Range&& range) {
  return Dune::iteratorTransformedRangeView(std::forward<Range>(range), [](auto&& it) {
      return std::tuple<decltype(*it), decltype(it.index())>(*it, it.index());
  });
}

...

for(auto&& [entry,col] :  sparseRange(matrix[row]))
  y[row] += entry*x[col];

```

Some more implementation details:

* `TransformedRangeView` has a new template parameter for selecting
  value- or iterator-based transformation. Value-based is default.
* A new helper function `iteratorTransformedRangeView()` allows
  to generate iterator based transformations.

Merge branch 'feature/find_gmp' into 'master'

ref:core/dune-common\>

### Summary

This MR adds imported-targets to the FindGMP.cmake module, following modern
cmake guidelines.

### Details

Instead of exporting include directories and libraries as variables, the find
modules exports an imported target that contains all the necessary information
to include and link GMP. Two such imported targets are provided: GMP::gmp and
GMP::gmpxx for the C and C++ library, respectively.

### Acknowledgements

This find-module is developed together with @lukas.riedel

See merge request [core/dune-common!818]

  [core/dune-common!818]: gitlab.dune-project.org/core/dune-common/merge_requests/818

Merge branch 'feature/find_quadmath' into 'master'

ref:core/dune-common\>

### Summary

This MR adds imported-targets to the FindQuadMath.cmake module, following
modern cmake guidelines.

### Details

Instead of exporting include directories and libraries as variables, the find
modules exports an imported target that contains all the necessary information
to include and link QuadMath. The imported target is named:
QuadMath::QuadMath.

### Acknowledgements

This find-module is developed together with @lukas.riedel

See merge request [core/dune-common!819]

  [core/dune-common!819]: gitlab.dune-project.org/core/dune-common/merge_requests/819

Move HAVE_GMP and dune_register_package_flags to AddGMPFlags

Link GMPxx library to C library GMP

Move HAVE_QUADMATH and dune_register_package_flags to AddQuadMathFlags

Merge branch 'cleanup/fix-deprecation-warning-lcm-gcd' into 'master'

See merge request [core/dune-common!826]

  [core/dune-common!826]: Nonecore/dune-common/merge_requests/826