Previously, there was a list of special matrix types that ended
the recursion (FieldMatrix, DiagonalMatrix, etc.).  Simply
ending the recursion at number types simplifies the code
considerably, and allows to write matrices like BCRSMatrix<double>
and such.

This allows to call writeMatrixToMatlabHelper for matrix types
that do not actually implement true matrix rows, such as
DiagonalMatrix and ScaledIdentityMatrix.

Merge branch 'implement-ilu-double' into 'master'

ref:core/dune-istl This merge request shifts to a new implementation strategy:
There are methods toVector and toMatrix now that interpret scalars as
FieldVector or FieldMatrix objects, respectively. That allows to get rid of
the Hybrid::ifElse pattern, which is legible but longer.

See merge request [!264]

  [!264]: gitlab.dune-project.org/core/dune-istl/merge_requests/264

Merge branch 'feature/fgmres' into 'master'

ref:core/dune-istl Implements a right-preconditioned flexible GMRes solver.

See merge request [!98]

  [!98]: gitlab.dune-project.org/core/dune-istl/merge_requests/98

Merge branch 'feature/gmres-fix-prec-call-zeroresidual' into 'master'

ref:core/dune-istl The post-processing routine was called twice, once before
the outer while loop and once after when the residual was initially zero. This
lead to a segmentation fault in the AMG preconditioner. Now it only gets
called once.

See merge request [!172]

  [!172]: gitlab.dune-project.org/core/dune-istl/merge_requests/172

Merge branch 'fix/gmres-dont-restart-on-last-iteration' into 'master'

ref:core/dune-istl Kudos to @mathis.springwald for pointing this out.

See merge request [!245]

  [!245]: gitlab.dune-project.org/core/dune-istl/merge_requests/245

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