Do not create a temporary copy of the matrix even if the ignoreNodes bitset is not empty

This save some memory when the matrix is large.  The dune-istl UMFPack solver
(which we use internally here) actually has support for sending only submatrices
to UMFPack proper.  Unfortunately, this feature is not well-documented...
Also, it only works if all dofs in a block-vector block are either completely
ignored or completely un-ignored.