Update basix.ufl.mixed_element factory function for case of a single element

[garth-wells]

With this change, a call to basix.ufl.mixed_element with a list of just one finite element simply returns the single element.

This allows an element to be 'constructed' from a list of one element without making a single element into a mixed element (when it's not a mixed element).

[nate-sime]

This proposed resolution doesn't seem like the natural way to deal with the problem.

I'd like a mixed element constructed as implemented in the DOLFINx library. I don't want Basix to give me unexpected behaviour returning the element I provide as an argument. The interface should be explicit.

The returned type will not be a mixed element in all cases as stated in the documentation.

[RemDelaporteMathurin]

Thanks for looking into this @garth-wells

FWIW I agree with @nate-sime in the way that we would like a "mixed element" of only one element to behave like an actual MixedElement. The problem isn't only at the construction level but also later on when used.

For example I want to be able to call ufl.split on a function of a functionspace built with that "mixed" element. I believe the proposed interface would not make this possible, which means I need to have different ways to handle things depending on the number of sub-elements, which defeats the purpose imo.

[garth-wells]

This proposed resolution doesn't seem like the natural way to deal with the problem.

I'd like a mixed element constructed as implemented in the DOLFINx library. I don't want Basix to give me unexpected behaviour returning the element I provide as an argument. The interface should be explicit.

The returned type will not be a mixed element in all cases as stated in the documentation.

We need a statement of needs rather than likes. What functionality is needed (preferably with concrete examples)? We don’t want to bogged down with semantics. We need to be precise on the concepts. My starting point is that the main distinction between an element and a mixed element is a well-defined value shape.

[RemDelaporteMathurin]

What functionality is needed (preferably with concrete examples)?

Sure, I tried to be more specific in this comment festim-dev/FESTIM#922 (comment) although this is hard to parse because it's FESTIM-specific. I'll try to make a pure dolfinx example to show how we use this

EDIT: I added an example here FEniCS/dolfinx#3519 (comment)