Previously, all code was in the specialization of that class
for BCRSMatrix<FieldMatrix>.  That was overly restrictive:
most of the code does not actually rely on the precise type
of the matrix.  As a first step towards allowing more general
matrix type we move all the code into the general class.

In the debug output, 'n' was used for the number of matrix columns.
However, 'n' is the number of columns of the FieldMatrix objects
that serve as matrix entries.

Merge branch 'implement-preconditioners-with-scalar-matrix-entries' into 'master'

See merge request [!269]

  [!269]: Nonecore/dune-istl/merge_requests/269

Merge branch 'implement-printmatrix-matrix-double' into 'master'

See merge request [!268]

  [!268]: Nonecore/dune-istl/merge_requests/268

Merge branch 'implement-printmatrix-for-scalar-entries' into 'master'

See merge request [!267]

  [!267]: Nonecore/dune-istl/merge_requests/267

Simplify the implementation while we're there.  The IsNumber trait
allows much simpler recursion termination.

The code still involves recursion, therefore the separate 'Helper'
method remains necessary.

Merge branch 'minor-doc-fixes' into 'master'

See merge request [!266]

  [!266]: Nonecore/dune-istl/merge_requests/266

Merge branch 'implement-matlab-writing-for-matrices-with-scalar-entries' into 'master'

See merge request [!265]

  [!265]: Nonecore/dune-istl/merge_requests/265

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