Merge branch 'feature/completeFCGSolver' into 'master'

ref:core/dune-istl I have modified the FCGSolver a little bit so that it's
easier to handle, and also renamed it for clarity to RestartedFCGSolver. Than
I derived the CompleteFCGSolver from it.

The CompleteFCGSolver uses as many old search directions as possible. This
results in more stability, but also in more average memory consumption.

See merge request [!237]

  [!237]: gitlab.dune-project.org/core/dune-istl/merge_requests/237

Merge branch 'implement-matrixmarket-for-bcrsmatrix-double' into 'master'

See merge request [!259]

  [!259]: Nonecore/dune-istl/merge_requests/259

Merge branch 'do-not-use-std-iterator' into 'master'

ref:core/dune-istl It was deprecated in C++17.

See merge request [!257]

  [!257]: gitlab.dune-project.org/core/dune-istl/merge_requests/257

Merge branch 'fix-typos' into 'master'

See merge request [!258]

  [!258]: Nonecore/dune-istl/merge_requests/258

Merge branch 'generalize-nonzerocounter-to-scalar-matrices' into 'master'

See merge request [!252]

  [!252]: Nonecore/dune-istl/merge_requests/252

The new code uses Dune::IsNumber to end the recursive counting.
Incidentally, this removes one more of the few uses of the
'blocklevel' mechanism.  It also removes one level of indirection
within the implementation.

It was deprecated in C++17.

Merge branch 'fix-dotproducttest-on-libc++' into 'master'

See merge request [!254]

  [!254]: Nonecore/dune-istl/merge_requests/254


Closes #51

Merge branch 'doc/fix-typos' into 'master'

See merge request [!255]

  [!255]: Nonecore/dune-istl/merge_requests/255

On clang-7 with libc++ this results in a small error for the imaginary
unit which gets computed as `I = (-4.37114e-08,1)`.  This results in a
too large error in the tests.

Closes: #51

Merge branch 'bugfix/fix-vbv-create-iterator' into 'master'

ref:core/dune-istl When the postfix increment operator is used, block sizes
will not be set properly, see [#56].

This implementation places the allocation into the destructor of the iterator.

It does not need additional structures.

This MR also fixes the test for interoperability with STL algorithms: As the
CreateIterator is only an output iterator, use std::fill_n() instead of
std::fill().

See [!249]. Fixes [#56].

See merge request [!250]

  [#56]: gitlab.dune-project.org/NoneNone/issues/56
  [!249]: gitlab.dune-project.org/NoneNone/merge_requests/249
  [!250]: gitlab.dune-project.org/core/dune-istl/merge_requests/250


Closes #56

Tunnel size access of the CreateIterator through a proxy to work around interface limitations in the window type

See issue #56.

std::fill() requires a forward iterator, so don't use that algorithm.

Merge branch 'ci/centralize-config' into 'master'

See merge request [!248]

  [!248]: Nonecore/dune-istl/merge_requests/248

Merge branch 'feature/cleanup-dotproducttest' into 'master'

ref:core/dune-istl

-   Use Dune::Std::negation to negate meta function
-   Indent Dune::Hybrid::ifElse branch for better readability

See merge request [!247]

  [!247]: gitlab.dune-project.org/core/dune-istl/merge_requests/247

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

ref:core/dune-istl

-   Avoid a sign/unsigned comparison warning
-   Don't use C's unqualifies size_t
-   Use std::size_t in 0,...,k-1 loop

See merge request [!246]

  [!246]: gitlab.dune-project.org/core/dune-istl/merge_requests/246

* Use Dune::Std::negation to negate meta fuction
* Indent Dune::Hybrid::ifElse branch for better readability

* Avoid a sign/unsigned comparison warning
* Don't use C's unqualifies size_t
* Use std::size_t in 0,...,k-1 loop

Merge branch 'allow-blockvector-of-doubles' into 'master'

ref:core/dune-istl So, for example, you can write

    BlockVector<double>

instead of

    BlockVector<FieldVector<double,1> >

and it will do what you expect it to do. With this patch, any type for which
Dune::IsNumber<T>::value returns true will end the recursion.

See merge request [!240]

  [!240]: gitlab.dune-project.org/core/dune-istl/merge_requests/240