Add shiftedBy method for covariance matrix

Once the changes of #971 (closed) finished, it could be interesting to add a method shiftedBy in StateCovariance. The implementation can have the same behaviour as the well known shiftedBy methods in Orekit (i.e., supposing keplerian motion). The state transition matrix can be computed analytically like it is done in UnscentedSemiAnalyticalKalmanModel.