remove_gates can now delete the last occurence of a given gate as well
remove_gates can now delete the last occurence of a given gate as well

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