Improve the error message in the DenseMatrixAssigner for DiagonalMatrix

Simon Praetoriusrequested to merge
issue/wrong-warning-diagonalmatrix-assigner into master

Summary

The DUNE_ASSERT_BOUNDS macro is misused in the DenseMatrixAssigner for DiagonalMatrix. The error message we get is "Index out of bounds", but we meant "Incompatible dimension in assignment". This is fixed in this MR.

Merge request reports