Draft: PrTrackAssociator , protect the cumulation of associations to be enabled for DecayVertex only

FYI : @decianm , @graven , @rjhunter I opened this MR to discuss what exactly we need to consider for the track associator table creation concerning decay in flight, shower of tracks from material interaction and any secondaries 'type' of hits produced from a charged track.

This is because I noticed that the old Run1/2 and the latest TrackAssociator (deleted in !3435 (merged) ) which I was using in the past (some years ago) had some protection on the VertexType to be of Decay type when merging hits for the association. The latest PrTrackAssociator code doesn't. Do you know why? If the merging of hits of daughters to mothers has to accomodate only decay type matching ( such as decay in flight ), wouldn't be better to put the condition in the code?