print warning when off-diagonal element is found in Schur complement instead of OPM_THROW.
Merge remote-tracking branch 'upstream/master'
bugfix, Mismatched free() / delete / delete [] in DuneMatrix due to use of new instead
of the matrix internal allocators.

This fix also avoid the copying of the BCRSMatrix by providing a contructor that creates
the DuneMatrix for a given Eigen SparseMatrix.
Merge pull request #218 from dr-robertk/dunematrix

Dunematrix uses wrong allocation.