Improve docstring in states

[Ericgig]

Description
As presented in #1722, the docstring of the states and operators functions are not consistent.

This change the docstring so that Number of basis states in Hilbert space is used everywhere, instead of fock states or vector states.

This set the variable names as:

• Single Hilbert space function : N
• Multiple Hilbert space function: dimensions
• Number of qubits: N_qubit

Added support for dimensions input to take Space object.
Added sanity check in dimensions object.
Added scalar_like method to dimensions object. In transform any dims [[2, 3], [2, 3]] to it's scalar with same structure [[1, 1], [1, 1]].
Added another input parameter to qzero to create rectangular operator qzero([2],[3]).dims == [[2], [3]].

Related issues or PRs
fix #1184, replace #2075

[coveralls]

coverage: 85.91% (+0.1%) from 85.808%
when pulling f2baffd on Ericgig:feature.dims.in.creation
into 4f6231a on qutip:master.

[hodgestar]

I left a couple of small questions, but otherwise this looks good.

We should document the methods of the dimension and space classes better (i.e. add docstrings which describe the arguments) but happy for that to happen sometime in a later PR.

[hodgestar]

Looks good to me now. Removing _implicit_tensor_dimensions made everything much nicer.

Thanks for adding to the flatten docstring. I suggested wording a small change to the note at the end.

I also suggested some small tweaks to qzero which for me make the logic slightly clearer.