Uniform refinement of mixed topology meshes

[chrisrichardson]

Allows mixed topology meshes, e.g. triangle+quad, or hex+pyramid+prism+tet to be refined globally.

This also works for pure or quad hex meshes, for example.

[jorgensd]

I'm not quite done reviewing, but here are my initial comments.

[schnellerhase]

Should maybe the entry point for uniform_refine and the already present refine share a common API? refine already supports uniform refinements by marking of all edges, and in the future the refinement routines implemented here could be extended to non uniform cases.

Also the partitioner creation, especially for refinement aware identity_partitioner to come #3661, should be available for the uniform refinement. This would be easiest to accomplish if the entry point is a shared one.

[chrisrichardson]

About unifying the interface:
Yes, in principle, we can do that. At the moment, I see this as slightly "experimental", and there are a couple of rough edges to clean up. I think making a common entry point should come last.

For some near future PRs, we can look at:

• using the coordinate map to compute the "midpoints" of edges, etc. for higher-order input. We can't create higher-order output meshes easily, since that requires initialising edges on the new mesh.
• In the Python layer, setting the ufl_domain to something other than None
• Creating a Python wrapper outside of dolfinx.cpp
• Supply a custom partitioner