Add Form arity property

[schnellerhase]

There is currently no better way than len(form.arguments()) to check for the arity/rank of a form. Introduces property for simpler access. Since rank is already associated with tensor ranks in ufl, use the name arity.

Ref FEniCS/dolfinx#4118.

[jorgensd]

A form doesn’t necessarily have a slingle arity, as each individual integrand can have different arities. This is for instance why things such as ‘ufl.system’ exists, as it would split the form by rank 2 and rank 1 systems.

if we want to have arity as a property, it should probably be max arity.