Add functor to access PV tracks

This MR adds two new functors:

• PVTRACKS this returns the tracks used in a given vertex
• COUNT_IF(predicate) this functor counts the number of objects passing a requirement.

This allows information about the tracks used in the PV fit to be retrieved as follows:

• Number of PV tracks F.SIZE_OF @ F.PVTRACKS @ F.BPV(pvs)
• Kinematics of PV tracks F.MAP( F.PHI ) @ F.PVTRACKS @ F.BPV(pvs)
• Number of backwards tracks in PV F.COUNT_IF( F.TRACKISVELOBACKWARDS) @ F.PVTRACKS @ F.BPV(pvs)

Corresponding tests have also been introduced.