fixed factor 2 bug in covariance_matrix
fixed factor 2 bug in covariance_matrix

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