WIP: HLT1 data-oriented event model - VELO

See general data-oriented event model discussion here: Rec#48 (closed)

Replaces track v2's data in HLT1 with a data-oriented design. This implies separating the track into hits, states and additional data where applicable. Each new data object (i.e. VeloHit, UtHit, FtHit) will now be a plain old data structure (struct). This data-oriented model can trivially be further developed into a SOA model to enable efficient vectorization of HLT1 algorithms.

These changes only affect velo (pixel) clustering, pixel tracking, pat pv and the velo-ut.