Correct the communication in apply<forward> of NonoverlappingBlockPreconditioner
requested to merge simon.praetorius/dune-istl:issue/templated_apply_block_preconditioner into master
Summary
Change the communication method in apply<forward>() of NonoverlappingBlockPreconditioner from copyOwnerToAll to addOwnerCopyToOwnerCopy as in the non-templated apply() method.
This MR closes #78 (closed) and resolves a comment in !302 (merged) (where this bug was introduced)