Added fallback for VELO tracks' cov matrix in DTF (2018-patches)
Currently, the Kalman step in the DTF fails regularly for velo tracks. This will resolve it, partially. (still a good initial value for the momentum at the first state is advisable)
Now based on the 2018-patches branch (sorry about the previous MR, 2018-patches seems more suitable). Tagging again @wouter.
Edited by Laurent Dufour