Follow-up from "Add vertex constraints to DecayTreeFitterAlg"

The following discussions from !2676 (merged) should be addressed:

• @amathad started a discussion: (+5 comments)

  I see the name is taken from fct.name() and not user defined. Do we want to take user specified name in which case functors should be a dict or leave it as is?

• @amathad started a discussion: (+4 comments)

  I had some doubt (even in my MR) and would be glad if you could clarify here. If I have a decay with intermediate particles e.g. Bs -> Jpsi (->mu+ mu-) phi (-> K+ K-), does the m_descendants->descendants( p ) return all intermediate particles along with final state paritcles? i.e the full list (Jpsi, phi, mu+,mu-,K+,K-)?

• @ldufour started a discussion: (+1 comment)

  From a quick look, right now the best PV is considered. Is the design already allowing the next step, i.e. the information for all of the PVs, as we had in Run 1/2? Or what do you think needs to be done to allow for that?

  • Maybe !2682 (merged) can be re-used.

• can the DTF chi2 be returned? #423