better internal support for working with sparse matrices when calculating propagators
better internal support for working with sparse matrices when calculating propagators

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