Use variable sized vector field to store LHCbIDs and hit indices in PrTracks

• Add support for unique fields (SOA fields backed by an unique value)
• reverse singular and plural definitions of fields
• renamed NDNumericProxy -> NumericProxy to reflect its new usage
• use vector instead of array for hits in Velo tracks
• use vector instead of array for hits in Upstream tracks (split lhcbids in a vp and ut part)
• use vector instead of array for hits in Seed tracks
• use vector instead of array for hits in Long tracks (split lhcbids in a vp, ut and ft part)
• use vector instead of array for hits in Downstream tracks (split lhcbids in a ut and ft part)
• use vector instead of array for hits in tracks v3 (split lhcbids in a vp, ut and ft part)

Goes with Rec!2772 (merged)

@graven