Add functors for composite flight distance vector from BPV

The merge adds additional functors to get all vector components of the distance between a composite vertex and it's best primary vertex, and the magnitude of the distance, beyond the previously implemented BPVVDZ and BPVVDRHO.

This is potentially a step towards allowing more complex calculations like perpendicular momentum discussed https://gitlab.cern.ch/lhcb-dpa/project/-/issues/61#note_5004057 But it seemed best to first add the basic ingredients.

Testing in Moore selections will require additional facilities for handling vectors, I think?