Fix infinite recursion for dolfinx.fem.Constant.__float__() calls

[mikics]

This PR may solve the infinite recursion related to GH issue #2481 by adding __int__, __float__ and __complex__ methods directly to the dolfinx.fem.Constant class, instead of inheriting them from parents' classes.

[jhale]

Thanks for this! A couple of small points

1. Can you add some test coverage? I suppose we never call these methods in our tests, so the issue was never picked up.
2. Is the __int__ method necessary? Under what circumstances would it be called?

[mikics]

Sure, I can add some tests!

For the __int__ method: I checked my code, and indeed it is not needed to solve the bug. I will remove it.

[mikics]

Tests added!