Allow variant hints to be passed into VectorElement and TensorElement

[mscroggs]

This change allows the user to pass a variant into VectorElement and TensorElement. The value of this is then passed to the FiniteElement that is created as the sub element.

[wence-]

I think this is fine, though I also think it indicates that this is a part of UFL that really should be redone.

I don't know how basix represents Vector/TensorElement, but for us, we represent them as Vector(ScalarElement(...)) so the scalar element has the variant information.

I think that a redesign of UFL elements has an algebra such that a UFL element is either a concrete (non-reducible) element (such as RT or Lagrange), or some modifier of a (list of) elements.

That way we don't end up glomming so much information into the FiniteElementBase thing (and having to break the inheritance patterns).

[mscroggs]

I think that a redesign of UFL elements has an algebra such that a UFL element is either a concrete (non-reducible) element (such as RT or Lagrange), or some modifier of a (list of) elements.

This is pretty much exactly how we treat elements in DOLFINx and FFCx now. I totally agree that this needs refactoring at some point.