Correct the communication in apply<forward> of NonoverlappingBlockPreconditioner

Simon Praetoriusrequested 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)

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