Draft: Use current perigee when recomputing perigee for TRT standalone tracks.

The perigee of TRT standalone tracks has a corrected covariance. The other track states have en optimistic covariance matrix. The extrapolation may favour other track states as a starting point for the extrapolation to the beam-line etc. . As a consequence the resulting extrapolated track parameters may be very different.

By enforcing that the original perigee is used for the extrapolation, the resulting covariance matrices are consistent and will be based on the more realistic covariance matrix of the original perigee.