Merge branch 'fix_matrixredistribute_for_scalar_valued_matrices' into 'master'

ref:core/dune-istl This MR adapts matrixredistribute.hh for matrices with
scalar block_type like BCRSMatrix<double> and adapts the
matrixredistributetest.

See merge request [core/dune-istl!278]

  [core/dune-istl!278]: gitlab.dune-project.org/core/dune-istl/merge_requests/278