Added checks for parameter values. Closes #66.
Added checks for parameter values. Closes #66.

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