Update on track finding in FDC

The segment-based track-finding algorithm has been refined:

• I no longer rely on the refitting to determine the charge of the particle unless the individual segment information is ambiguous for those segments linked into candidates.
• The dip angle and vertex position are determined from either the most upstream segment or the result of the refit depending on which gives a vertex position closest to the center of the target.
• The track-matching cut from segment to segment is now momentum-dependent: $\delta r<2.79+2.88/p^{2}$.