Merge branch 'fix/non-mpi-build' into 'master'

ref:core/dune-istl\> When MPI is not available or explicitly disabled with the
CMake build option -DCMAKE_IDSABLE_FIND_PACKAGE_MPI=TRUE, some tests were
unable to build. I've either fixed or disabled those tests (for those tests
that were testing MPI-only features).

I've also fixed overlappingschawarztest, which did not build when SuperLU was
not found.

The commit concerning MPI should probably be backported to the releases/2.7
branch, as it is affected by the same build failures. The fix for
overlappingschwarztest only appeared after commit
05f80ffa which fixed a bug which previously
disabled the test. Since this commit is not on the releases/2.7 branch, there
is probably no need to backport my fix for overlappingschwarztest.

See merge request [core/dune-istl!372]

  [core/dune-istl!372]: gitlab.dune-project.org/core/dune-istl/merge_requests/372

This test should not be necessary anymore, since commit
05f80ffa fixed the CMAKE_GUARD argument
for this test, so that it is not compiled anymore if either UMFPACK or
SuperLU are missing.

That line had to be removed as it did not compile if either UMFPACK or
SuperLU were missing. For example while I had UMFPACK installed but not
SuperLU, gcc gave me the following error message for the removed line:

…/dune/istl/test/overlappingschwarztest.cc:260:17: error: ‘HAVE_SUPERLU’ was not declared in this scope

Merge branch 'feature/addPythonBindings' into 'master'

See merge request [core/dune-istl!369]

  [core/dune-istl!369]: Nonecore/dune-istl/merge_requests/369

When MPI is not available or explicitly disabled with the CMake build
option -DCMAKE_IDSABLE_FIND_PACKAGE_MPI=TRUE, some tests were unable to
build.

The tests created from solverfactorytest.cc.in and part of
scalarproductstest.cc use Dune::OwnerOverlapCopyCommunication which is
defined behind `#if HAVE_MPI` and is thus not available in a non-MPI
build. I've thus disabled those tests when MPI is unavailable.

The matrixmarkettest did not work without MPI, as it contained some code
using the wrong template parameters when HAVE_MPI was not set. Those
template paramters have been fixed now.

I've confirmed, that after my changes `make build_tests` succeeds to
build all tests and that those tests run without failure.

Python bindings, default is OFF.

Merge branch 'feature/cholmod-common' into 'master'

ref:core/dune-istl\> This allows other implementations that are using the
istl-cholmod wrapper to set options and other parameters of the cholmod
solver.

See merge request [core/dune-istl!366]

  [core/dune-istl!366]: gitlab.dune-project.org/core/dune-istl/merge_requests/366

Merge branch 'bugfix/cmake_guard_condition_overlappingschwarztest' into 'master'

ref:core/dune-istl\> This MR fixes the CMAKE_GUARD condition for the
overlappingschwarztest.

See merge request [core/dune-istl!367]

  [core/dune-istl!367]: gitlab.dune-project.org/core/dune-istl/merge_requests/367

This allows other implementations which are using the istl-cholmod wrapper
to set options and other paramters of the cholmod solver

Merge branch 'feature/parallel-solver-factory' into 'master'

ref:core/dune-istl\> This is my proposal for supporting the parallel solvers
in dune-istl with the solver factory.

Summary of the changes:

-   The parallel operators let one get the underlying communication object.
    This is needed to pass it to the parallel preconditioners
-   The solver factory passes the Operator instead of the matrix to the
    preconditioner factory. Thus AMG can be constructed without creating
    another operator from the matrix and we can determine whether we are
    parallel or not by the operator.
-   For the parallel solvers we need to initialize the factory with the
    corresponding parallel operator.

Note that further refactoring might be required to support e.g. the
grid-communcation-based parallelization approaches sometime used in PdeLab.

This is one part of the work proposed in [#82]

See merge request [core/dune-istl!349]

  [#82]: gitlab.dune-project.org/NoneNone/issues/82
  [core/dune-istl!349]: gitlab.dune-project.org/core/dune-istl/merge_requests/349

Currently the hierarchy building does not work for complex.

```
dune/istl/schwarz.hh:280:12: warning: struct 'ConstructionTraits' was previously declared as a class [-Wmismatched-tags]
    friend struct Amg::ConstructionTraits<BlockPreconditioner<X,Y,C,P> >;
            ^
```

It is the linear operator anyway. Hence we can deduce the rest.

We need to use the correct discretization for this. Using
the overlapping one we are getting nans.

Of course all ranks need to participate in the computation.

This is a preparation for the parallel solver factory and will allow
to construct AMG from the ptr passed to the factory.

Merge branch 'feature/remove-more-deprecated-stuff-after-2.7' into 'master'

See merge request [core/dune-istl!364]

  [core/dune-istl!364]: Nonecore/dune-istl/merge_requests/364

It fails when SuperLU or one of the SuiteSparse
libraries are missing.
We should develp a more fine-grained solution.