ENH: Optimized cython for sparse_utils (#568)
ENH: Optimized cython for sparse_utils (#568)

* Optimized cython for sparse_utils

* minor notation change

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