added convenience function lindblad_dissipator for cases when not the full liouvillian is needed, but only the lindblad dissipator for a single collapse operator
added convenience function lindblad_dissipator for cases when not the full liouvillian is needed, but only the lindblad dissipator for a single collapse operator

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