[bugfix] matrixredistribute for matrices with scalar block_type and add a test

Nils-Arne Dreierrequested to merge
nils.dreier/dune-istl:fix_matrixredistribute_for_scalar_valued_matrices into master

This MR adapts matrixredistribute.hh for matrices with scalar block_type like BCRSMatrix<double> and adapts the matrixredistributetest.

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