Distinguish interior and ghost elements during parallel execution

[IshaanDesai]

Currently the adapter does not distinguish between interior elements and overlap/ghost elements when it defines vertices as coupling vertices in preCICE. It simply passes the coordinates that the user provides. In precice/tutorials#663 the macro-dumux participant code ensures that only interior elements are used while setting vertices and passing data to and from preCICE. Although the application code has handled this for now, it would be more appropriate to handle this in the adapter in a more general way.

[mathiskelm]

For elements the distinction is clear where interior elements (tagged as such on exactly one MPI rank) are communicated to preCICE by their rank and data for overlap/ghost elements is exchanged with their home rank. This can be done by using Dune::Partitions and gridView.communicate() with DuMux data handles as done in the tutorial case.

For coupling vertices on the boundary between the partitions (interior region of ranks, DUNE partition of border) one of the ranks should take responsibility and exchange data with adjacent ranks (who have to take care not to write their invalid values).