Fixed Gaussian2D model when a cov_matrix is input

This PR fixes the rotation direction of the `Gaussian2D` model when a `cov_matrix` is input.  `theta` is calculated from `cov_matrix` as the angle of the major-axis eigenvector using `np.arctan2()`, which returns a *counterclockwise* angle relative to the positive `x` axis.  However, the `theta` used by `Gaussian2D` is defined in a *clockwise* direction.  This PR corrects the rotation direction.