[!289] Simplify code using asMatrix()

Merge branch 'feature/implement-scalarmatrixview' into 'master'

ref:core/dune-istl This is based on [dune-common!656] which introduces
Impl::asMatrix() using this we can avoid many Hybrid::ifElse constructions
which increase complexity of the implementation and error messages.
Furthermore we can drop toMatrix() which fixed [#63].

This is a clone of [!288]. Is was created in order to tell the CI to use the
corresponding branch in dune-common using a matching branch name.

See merge request [!289]

  [dune-common!656]: gitlab.dune-project.org/Nonedune-common/merge_requests/656
  [#63]: gitlab.dune-project.org/NoneNone/issues/63
  [!288]: gitlab.dune-project.org/NoneNone/merge_requests/288
  [!289]: gitlab.dune-project.org/core/dune-istl/merge_requests/289


Closes #63