I posed this MR into the box-facet-coupling branch so that the diff is easier to read. If we wait for !1492 (merged), which itself has to wait for !1431 (merged), then we can merge this into master at one point.

@timok, @kweis, maybe you can have a look at this. If we want to keep this, we would have to:

• deprecate old interfaces
• change all tests where cm now receives grid variables
• create an issue to remove the deprecated interfaces in all facet coupling managers and the poroelasticcouplingmanager until version 3.2 release (edit Timo: don't need an issue for that)
• update reference to test_md_facet_tracer_box (velocity is included now, which tests the MD vel output)
• Put a note in the changelog that scvfs now have to implement neighbor()

Fixes #619.