Checks for `is_separable` are not complete

[vprusso]

Several computational conditions are checked to determine if a state is separable presently in is_separable.py. However, several items are still not covered compared to the IsSeparable function from QETLAB.

This issue covers the additional separability checks from IsSeparable and includes test cases for each condition.

Note: At present, if all other separability checks are not hit, the failsafe is the final separability check imposed by invoking the symmetric extension hierarchy. This will eventually converge and will determine whether the state is separable (however this could be computationally intractable in general, and ideally, hitting as many of the prior cases first is ideal).

With that in mind, not all separability checks from QETLAB need to be ported over, but adding some more of them into toqito would be ideal. This issue could be considered closed then if a subset of the separability checks are ported over.