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.

Only SuperLU 5 is now supported.

Merge branch 'fix-typo' into 'master'

See merge request [core/dune-istl!362]

  [core/dune-istl!362]: Nonecore/dune-istl/merge_requests/362

Merge branch 'bugfix/ConstructionTraits' into 'master'

ref:core/dune-istl Redefine all remainig class definitions as structs.

See merge request [core/dune-istl!361]

  [core/dune-istl!361]: gitlab.dune-project.org/core/dune-istl/merge_requests/361

Merge branch 'fix/amg-verbosity-default' into 'master'

ref:core/dune-istl A default verbosity of 2 clutters everything with a lot of
output, per default.

See merge request [core/dune-istl!360]

  [core/dune-istl!360]: gitlab.dune-project.org/core/dune-istl/merge_requests/360