StateTomographyAnalysis: Remove the default usage of covariance matrix in LS tomography
StateTomographyAnalysis: Remove the default usage of covariance matrix in LS tomography

- Least square inverted the experimental covariance matrix,
which can be singular and therefore make the LS fit fail. The covariance
matrix can still be used if the corresponding flag is True.

QudevETH/PycQED_py3