investigate hit error rescaling to deweight outliers

[slava77]

Motivation: moderate outlier hits can distort the trajectory (after update) during building.

The idea is to rescale the hit covariance by a function of the initial hit chi2. The function could just be 1+ chi2 or something smoothed extending to around 2 sigma equivalent, e.g. chi2 < 4 ? 1 : chi2-3 (to be a continuous function).
The rescale would be applied in the Kalman update math and should be straightforward during search where we compute chi2 first and then run the update.