Allow for product of continuous Hilbert spaces

Created by: PhilipVinc

Right now we can only combine discrete Hilbert spaces, not continuous.

This PR extracts the common logic of tensorHilbert in a new AbstractTensorHiblert class, and creates two implementations

• TensorHilbert which is a combination of continuous Hilbert spaces, with no special new methods added
• TensorDiscreteHilbert which combines discrete spaces and is discrete itself, possibly even indexable. I've cleaned up the logic used in multiplying hilbert spaces so that you always get the correct type (a TensorDiscrete as long as you only have discrete spaces, the continuous one otherwise)

I'm unsure about the naming, and I'd like some feedback about how to name those three objects (AbstractTensorHilbert, TensorHilbert and TensorDiscreteHilbert).

In theory one should never have to construct directly a Tensor***Hilbert, but might only want to check for AbstractTensorHilbert in some cases.

This also renames TensorHilbert to TensorDiscreteHilbert which might break some user code, though I suspect few are using this directly (@jwnys ?)