Add degree argument to tensor product element reconstruction

[rckirby]

In using low-order preconditioners, we need to reconstruct a finite element with a lower-degree instance of the same element family. This PR enables this for tensor product elements.

[rckirby]

Pablo's PMGSNES PR over on Firedrake does a lot of what I need here, and it makes sense to work in one place at a time. His is more general, and we should try to migrate that over here once it lands. So I'm converting this to draft for the time being.