functionality for accessing subspaces of states and operators

[aarontrowbridge]

i don't see this in the docs for the python qutip package, but it would be very handy to have access to subspaces of quantum objects, c.f. this docs page in QuantumToolbox.jl.

in QuantumCollocation.jl we have constructed an EmbeddedOperator object for handling operators embedded in subspaces. we also frequently call a function a get_subspace_indices to index into the subspace of quantum states -- this functionality does not currently exist in QuantumOptics.jl, where also the tensor product ordering does not match what we (and you) have currently implemented.