* setSolverCategory is not allowed to alter a const value
* Adjust type of setSolverCategory's set
* Properly document deprecation of get/setSolverCategory

Merge branch 'feature/bump-to-2.8' into 'master'

See merge request [!342]

  [!342]: Nonecore/dune-istl/merge_requests/342

Merge branch 'issue/extra_semicolon' into 'master'

ref:core/dune-istl This is a small MR removing unnecessary semicolon after
some functions recently introduced by the solver creators. These semicolon
produce lots of compiler warnings when compiled with -pedantic

See merge request [!343]

  [!343]: gitlab.dune-project.org/core/dune-istl/merge_requests/343

Merge branch 'fix/solverfactory_parameters_of_default_iterative_solver_factory' into 'master'

ref:core/dune-istl ICC complains about using decltype(tl) in argument type
where tl is the first argument declared as auto.

See merge request [!341]

  [!341]: gitlab.dune-project.org/core/dune-istl/merge_requests/341

Merge branch 'bugfix/amg-tautological-compare' into 'master'

ref:core/dune-istl small fix t avoid compiler warnings and properly handle
range checking for potentially unsigned int values\...

See merge request [!339]

  [!339]: gitlab.dune-project.org/core/dune-istl/merge_requests/339

Merge branch 'fix/counter_overloads_namespace_clash' into 'master'

ref:core/dune-istl The namespace Dune::<unnamed>::Overloads collides with
Dune::Simd::Overloads and is not necessary as we are in an anonymous namespace
anyway.

See merge request [!340]

  [!340]: gitlab.dune-project.org/core/dune-istl/merge_requests/340

incCounter to DUNE_INC_COUNTER
getCounter to DUNE_GET_COUNTER

ICC complains about using `decltype(tl)` in argument type where `tl` is the
first argument declared as `auto`.

Merge branch 'feature/solverrepository' into 'master'

ref:core/dune-istl This MR introduces a solver factory that can assemble
solvers to a given LinearOperator, that are fully configurable via a
ParameterTree. It is based on a compile-time registry that can register class
templates at compile time. This compile time registry can then be added to a
ParameterizedObjectFactory.

We've introduced tree types of these Factories:

-   DirectSolverFactory
-   PreconditionerFactory
-   IterativeSolverFactory

We provide Macros for registering class templates. For example a
preconditioner can be registered like that:

    DUNE_REGISTER_PRECONDITIONER("ssor", default_preconditoner_BL_creator<Dune::SeqSSOR>());

where default_preconditoner_BL_creator<Dune::SeqSSOR>() could also be replaced
by a object that creates the preconditoner from a matrix and a ParameterTree.

The solver factory can then be used like this:

    std::shared_ptr<MatrixAdapter<Vector,Vector>> op = std::make_shared<MatrixAdapter<Vector,Vector>>(A);
    std::shared_ptr<Dune::InverseOperator<Vector, Vector>> solver = getSolverFromFactory(op, solverConfig);

where solverConfig is a ParameterTree containing a configuration tree:

    [CGWithAMG]
    type = cgsolver
    verbose = 1
    maxit = 1000
    reduction = 1e-5

    [CGWithAMG.preconditioner]
    type = amg
    iterations = 1
    relaxation = 1
    maxLevel = 10

It is planned to have a library "libduneistl" at some point that contains
preconditioners and solvers for the most often used template parameters.

See merge request [!312]

  [!312]: gitlab.dune-project.org/core/dune-istl/merge_requests/312

Merge branch 'check-mtbm-with-heterogeneous-nesting' into 'master'

ref:core/dune-istl Meaning: you need multi-indices of different length to get
the different scalar entries.

See merge request [!283]

  [!283]: gitlab.dune-project.org/core/dune-istl/merge_requests/283

Meaning: you need multi-indices of different length to get the
different scalar entries.

Merge branch 'fix/dont_use_deprecated_communication_headers' into 'master'

ref:core/dune-istl Unclear target: 2.7 or after the branching.

See merge request [!338]

  [!338]: gitlab.dune-project.org/core/dune-istl/merge_requests/338

Merge branch 'doc-prec-pre-apply-post' into 'master'

ref:core/dune-istl fixes [#67]

See merge request [!337]

  [#67]: gitlab.dune-project.org/NoneNone/issues/67
  [!337]: gitlab.dune-project.org/core/dune-istl/merge_requests/337


Closes #67

Merge branch 'make_matrixmarket_less_verbose' into 'master'

ref:core/dune-istl The matrixmarket functions send their output directly to
std::cout. This MR changes this to Dune::dverb.

See merge request [!336]

  [!336]: gitlab.dune-project.org/core/dune-istl/merge_requests/336

Merge branch 'fix-typos-in-comments' into 'master'

See merge request [!335]

  [!335]: Nonecore/dune-istl/merge_requests/335

with LoopSIMD

solver or preconditioner

customly created anyway