Fix error when propagating covariance expressed in a non-inertial frame

Hi everyone,

Opening this issue after @brevelin mentioned this problem on the following forum thread.

The error is cause by the init() method of StateCovarianceProvider which always call the changeCovarianceType() method of StateCovariance. When a StateCovariance is defined using a non-inertial frame, the changeCovarianceType() directly return an error even when the user/code tries to change the type from Cartesian to Cartesian.

I'll use this issue to fix some comments inside the StateCovariance class.

Cheers, Vincent