vs.to_qobj is broken for constrained Hilbert spaces

[PhilipVinc]

Qutip does not know about constrained Hilbert spaces, and only operates on 'full' spaces.
If you have a constrained Hilbert, we will build a QOBJ with the dimensions of the full space but we pass a vector with the size of the constrained one.

To fix this we should create the larger to_array in the unconstrained space. The conversion can be done easily with states to number...