fixed thermal_dm to use truncated operator method by default.  Also changed coherent and coherent_dm from 'displacement' to 'operator'
fixed thermal_dm to use truncated operator method by default.  Also changed coherent and coherent_dm from 'displacement' to 'operator'

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