added optional number state offsets to oscillator mode operators
added optional number state offsets to oscillator mode operators

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