added unit tests for updated Qobj.expm method
added unit tests for updated Qobj.expm method

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