Add a dtype argument to all Hilbert.number_to_state and state_to_number methods

[PhilipVinc]

also all_states.

This will mirror the same dtype argument to random state.
Required to work well with single precision in some cases, or to lower memory consumption in others.

(This may suggest we should add a dtype argument to Hilbert spaces, and samplers should look at it unless overridden, but who knows....)