Add some MC Functors

Adds a MC_MOTHER functor for MC studies 🗿 and renames the VX and X_ functor (and friends) to be more clear that it returns the end/origin vertex of a (MC)Particle.

Collections can be found in Analysis!846 (merged)

Test together with LHCb!3397 (merged), Moore!1304 (merged), Analysis!846 (merged) and DaVinci!646 (merged)

Waits for !2677 (merged), related to lhcb-dpa/project#145, also waits for LHCb!3367 (merged)

FYI @pkoppenb @erodrigu @amathad