code restructuring: move sp_exp to qutip.sparse. move pade function outside sp_expm
code restructuring: move sp_exp to qutip.sparse. move pade function outside sp_expm

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