bugfix, use axis=2 with np.sum when replacing python sum
bugfix, use axis=2 with np.sum when replacing python sum

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