Add copy-constructor and copy-assign to VariableSizeCommunicator

This basically implements the rule of 3, as we have a custom destructor. Without this patch, copying a VariableSizeCommunicator will lead to double frees.

This addresses issue #159 (closed). As stated there, an alternative might be to not use MPI_Comm_dup and MPI_Comm_free and thus not assuming any ownership at all. Then, the default constructors etc. are probably just fine.

assigned to @markus.blatt

changed milestone to %DUNE 2.7.0

@carsten.graeser, I assume Markus won't have a look. Should we merge and cherry-pick for 2.7 anyway?

I don't see a reason not to merge... we might ask @nils.dreier as he has been experimenting with the communicators lately.

mentioned in merge request !737 (merged)

mentioned in commit fcfd0eb4

I merge right before branching as the odds seems to be good.

Rebased version merged as !737 (merged).

closed