ADDED: Position and momentum operators
ADDED: Position and momentum operators

Added functions for creating the position and momentum operators from
the linear combination of creation and destruction operators.

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