added single-mode displacement operator: displace(N,alpha)
added single-mode displacement operator: displace(N,alpha)

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