Add function to get the ufl.Cell corresponding to a cell's facet

[jpdean]

In some applications, it is useful to get the ufl.Cell corresponding to a cell's facets (i.e. for a triangle in 2D space, the facets are ufl.Cell('interval', 2)).

[chrisrichardson]

I've a feeling this already exidts as a free function, something like cell2facet - it's problematic with e.g. prism cells, because it depends which facet... can we do something better?

[chrisrichardson]

I am thinking of ufl.cell.cellname2facetname() which is used elsewhere in ffcx. It would be good to clean this up, rather than have two similar functions.

[chrisrichardson]

Looking at your PR, I see now that was a silly comment of mine, as you are using this function already. I'm still not happy about making use of something that is pretty flawed.

[jpdean]

OK, thanks! I'll look into improving cellname2facetname.

[chrisrichardson]

This looks good to me. @dham - can we delete ufl.cell.cellname2facetname? Or is this used in firedrake?

[chrisrichardson]

This is an improvement, which will allow us to remove the oddly named and inconsistent cellname2facetname function.