Tetmesh topology kernel: implement vertex_opposite_halfface()
Go from a halfface of a tet cell to the opposite vertex. This is currently possible with the local TetTopology, but that's overkill in many cases.
We also already have halfface_opposite_vertex()
to go from halfface to opposite vertex (although the naming is a bit confusing IMHO).
Ideally this would work for tet elements in mixed meshes too, not just in pure tet meshes.