Updates to the HSGNN code
This MR fixes technicalities on how the decorators where written, and updates to objects to vertex association used.
This MR updates the decorations from using the SG::AuxElement::Decorator concept to the decorator handle keys, given that as of a recent athena release deep copies of objects do not copy (unlocked) decorations.
In addition, the way in which the objects are associated to different PVs has been updated to use the TTVA tool, with the Prompt_MaxWeight WP: this assigns as best-matched-vertex the one for which the track has the largest AMVF weight.