Added docstrings to _isqubitdims and _cb_labels.
Added docstrings to _isqubitdims and _cb_labels.

qutip/qutip,zasdfgbnm/qutip,anubhavvardhan/qutip,cgranade/qutip,anubhavvardhan/qutip,qutip/qutip,zasdfgbnm/qutip,cgranade/qutip