Merge branch 'implement-AMG-for-scalar-valued-matrices' into 'master'

See merge request [!272]

  [!272]: Nonecore/dune-istl/merge_requests/272

To make it easier to understand.

Simplify the code while we're there.

Is not used.

Merge branch 'implement-umfpack-for-scalar-matrices' into 'master'

See merge request [!271]

  [!271]: Nonecore/dune-istl/merge_requests/271

To prepare for testing with more matrix types.

Merge branch 'implement-superlu-for-scalar-valued-matrices' into 'master'

See merge request [!270]

  [!270]: Nonecore/dune-istl/merge_requests/270

There is no reason why it should be a C-pointer.

Previously, only BCRSMatrix<FieldMatrix<T,1,1> > was tested.

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