Raise error when flipping continuous Hilbert

Created by: wdphy16

Previously if we use MetropolisLocal on a continuous Hilbert space, it will run into an infinite recursion, because both flip_state_scalar and flip_state_batch are not dispatched on ContinuousHilbert, and their default implementations just call each other.

Now we dispatch flip_state_scalar to raise an error, and we can state that MetropolisLocal only works with DiscreteHilbert.