Real element

[jorgensd]

From a basix point of view, a real-element is a DG-0 element. It is only DOLFINx that needs to know about this at dofmap construction ref (https://scientificcomputing.github.io/scifem/examples/real_function_space.html, https://github.com/scientificcomputing/scifem/blob/main/src/scifem.cpp#L60-L109)
Therefore, I don't see why we have https://github.com/search?q=repo%3AFEniCS%2Fbasix%20real_element&type=code
which I believe is untested and doesn't work with DOLFINx.

If it works it would be good to add a demo to DOLFINx with it.
If not, I think we should create a different kind of wrapper in basix to instruct DOLFINx to use a dofmap constructor similar to the one in scifem.