Merge branch 'SeqOverlappingSchwarz_scalar_matrices' into 'master'

ref:core/dune-istl This MR adapts the SeqOverlappingSchwarz preconditioner for
matrices with scalar block_type.

Is there a more elegant way of doing this:

    static constexpr size_t n = std::decay_t<decltype(Impl::asMatrix(std::declval<K>()))>::rows;

?

Feel free to push into the source branch!

See merge request [!306]

  [!306]: gitlab.dune-project.org/core/dune-istl/merge_requests/306