[forms] Separate LocalOperator and LocalFunction interface

So far a nullary LocalOperator tried to mimic a LocalFunction which is cumbersome, because both behave differently: The LocalFunction can simply be evaluated at points given by coordinates, while the LocalOperator requires and evaluation cache and a point given by its index in a quadrature rule.

Instead of pretending that both are the same and implementing them in one class, the new LocalFunctionAdaptor adapts a LocalOperator to the LocalFunction interface by managing a local cache.