Added qdiag operator
Added qdiag operator

Added operator constructed from a sequence of elements to be placed on
the diagonals. Requires SciPy 0.11+.

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