Fix outliers counting in PrKalmanFilter

Require minimum 3 real VP hits (but not projections) in outlier removal. Hopefully fixes rare errors seen in #275 (closed) but could be unrelated - needs testing.

(obviously, there are other ways to define a minimum number of hits)

Made TrackMasterFitter consistent with some clean-ups

@wouter @ldufour @chasse @gunther @valukash