Add elements of track covariance matrix to smart slimming list

This MR adds the elements of track covariance matrices to the smart slimming list in master. For reference, the full defining parameters were being added in 21.2 here - although this was before the EDM change to storing the diagonal and off-diagonal elements separately. Until now, no covariance matrix elements were being saved in master - probably related to the aforementioned EDM update and change in the accessor names.