PrKalmanFilter returning v3::Tracks and ExtraInfo and track fit results
This MR introduces new templates of PrKalmanFilter returning v3::Tracks and v3::PartialChiSquared (for ghost probability) and vector of full track fit results.
Includes some rationalisation of functions accessing hits of PrTrack.
Also includes a draft of v3->v1 track converter that can be used downstream of the track fit.
Will go with LHCb!3427 (merged) and Moore!1642 (merged) and Alignment!294 (merged) and LHCb!3366 (closed)