BUG: Incorrect return type when differentiating an `Action` with respect to a `Cofunction` about firedrake

[connorjward]

@nbouziani what are your thoughts?

Regarding the proposed inheritance change I think that that would be extremely disruptive in UFL, even if it is technically the right approach.

[jrmaddison]

Using a Function for the simplified derivative is also complicated by #3346.

[jrmaddison]

After some digging I think the relevant lines are 66-70 at https://github.com/FEniCS/ufl/blob/0f2cb2199476f9aaf98bbd942a22aa472ce5c639/ufl/action.py#L66. Removing these fixes the first case (after an update to _check_function_spaces).

I also see that this code has a double initialization bug -- if the returned right or left are themselves Actions then Action.__init__ will be called for them again (https://docs.python.org/3/reference/datamodel.html#object.__new__).