added a few more unit tests for qobj addition and multiplication
added a few more unit tests for qobj addition and multiplication

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