o use Vec3 for position and three-momentum, adapt accessors
o use unqualified dot for dot product
o more use of threeMomentum
o add slopes for those cases where the momentum magnitude is unknown or not required
o reduce duplication of code in PIDx functors
o remove no longer needed duck-typing accessors
o add referencePoint, endVertexPos and threeMomentum support to TrackCompactVertex