added support for calculating a sequence of propagator for consecuitive times
added support for calculating a sequence of propagator for consecuitive times

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