Functor that evaluates the FDCHI2 wrt a specific vertex

From a Mattermost chat on DPA WP3 Offline Analysis Tools group, it was discussed about the possibility to evaluate the flight distance \chi^2 with respect to a specific vertex, not the primary one.

After a chat with @amathad, maybe this functor could be built using the composition of different functors, like:

• One that retrieves the signal from the TES location with all the particles (like !3124 (merged))
• A functor that check somehow the relationship between the particles
• ENDVERTEX to get the vertex of the given particle
• VTX_FDCHI2 to evaluate the desired quantity

What is your opinion about it @graven ? Do you have some ideas/suggestions?

cc: @abertoli