added another unit test for partial transposes (comparing sparse and dense implementation)
added another unit test for partial transposes (comparing sparse and dense implementation)

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