Add utility to find sets of cells which are connected by facet in mixed topology

[chrisrichardson]

For dS integrals, we need to identify the sets of cells which are connected by each facet type, e.g. in a 3D mesh, the dS integrals for triangular facets may connect: tet-tet, tet-prism, tet-pyramid, prism-prism, prism-pyramid, pyramid-pyramid (6). For quads, the list is: hex-hex, hex-prism, hex-pyramid, prism-prism, prism-pyramid, pyramid-pyramid (6). In 2D, for interval facets, there is: quad-quad, tri-quad, tri-tri (3).

From the topology, we can create lists of shape [:, 2] listing the cell indices for each of the connectivities above.