Regroup Velo Kalman fits + numerical stability
See the motivation behind this in https://indico.cern.ch/event/786110/contributions/3537851/
This first goal of this MR is to regroup the code in PrPixelFastKalman.h and VeloKalmanHelper.h to share as many code as possible.
The second goal is to follow up on !1660 (merged) and implement the needed changes for numerical stability.
Requires Brunel!996 (merged) Moore!458 (merged) (ref updates)
Edited by Milosz Zdybal