changed entropy->entropy_vn to avoid conflict with matplotlib
changed entropy->entropy_vn to avoid conflict with matplotlib

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