Progress on 3D - indirect corner neighbors

[hannesbrandt]

We add domain_indirect_corner_neighbor both in 2D and in 3D. The function allows to query corner-neighboring ghost patches that reside on different processes, similar to domain_indirect_neighbors, which was renamed to domain_indirect_face_neighbors. The face and the corner information are communicated in a single ghost exchange in domain_indirect_begin and domain_indirect_end.

[cburstedde]

This looks solid!

If anybody sees a regression on the indirect face neighbors, please let us know.