Bugfix/cachedcomm lagrange

I think I observed a bug in the communication of (Lagrange) discrete functions. @robert.kloefkorn As far as I can see this backtraces to some changes you added, could you have a look?

On shared dofs, the DFCommunicationOperation, in particular, Add is executed multiple times for the same dof, leading to a wrong discrete function after communication (~cachedcommmmanager.hh:650). It's working fine for DG functions because every dof is touched only once, or if one (resp., the grid) ensures that every dof is only shared by exactly one interior and one ghost entity.

Is this a valid assumption? I try to parallelise MMesh atm and in general I will have multiple ghosts adjacent to an interior entity (think about networks).

I propose a bugfix checking if a dof has been communicated already, and this seems to fix the issue. Anything against this fix? Tested with test-comm in dune/mmesh/test/ on branch feature/mpi.