Filter mode for PrVeloUT

Introduces a filter mode in PrVeloUT, i.e. PrVeloUT checks for UT hits roughly matching the velo track and passes this track on as "filtered" upstream track. These upstream tracks do not contain UT hits, but can (and often will) have a momentum estimate. Using these tracks in the Forward Tracking reduces the number of input tracks and produced ghosts by a third, without compromising the efficiencies (modulo some missing tuning).

@decianm @sstahl @chasse @peilian @lohenry

Goes with LHCb!3216 (merged) and Moore!977 (merged)