add possibility of storing B decay time from external generator + various other edits

[dlanci]

This pull request aims at storing decay length and flight time information from external generators, e.g. If oscillation is enabled in EvtGen, B decay time can be saved to ntuple to include oscillation effects. It does so by using HepMCEvent as it is shown in EvtGen's examples (eg here)

In particular, this MR aims at:

1. Enabling definition of variables that will be written in the .DEC file with the syntax:

@0
    name : B0
    Define : dm_incohMix_B0 0.507e12, dgog 0.000, qoverp_incohMix_B0 1.00, argqoverp -0.775
    evtGenModel: SSD_CP dm_incohMix_B0 dgog qoverp_incohMix_B0 argqoverp 1.0 0.0 1.0 0.0

2. If dm_incohMix_B0 or dm_incohMix_B0s are defined this will enable B meson oscillations if your EvtGen model includes them

Test with the following decay and config files:

Bs0 -> {KS -> pi+ pi-} {Jpsi -> mu+ mu-}

geometry : 4pi
energy: 13

paramsDecaying : M, P, PT, PX, PY, PZ, E, vtxX, vtxY, vtxY, vtxZ, origX, origY, origZ, eta, FD
paramsStable : M, P, PT, PX, PY, PZ, E, IP, eta, MINIP


param : B0_ID ID 0
param : B0_HasOsc hasOsc 0
param : B0_decaytime decaytime 0
param : KS0_decaytime decaytime 1
param : JPsi_decaytime decaytime 2


useEvtGen : TRUE
evtGenUsePHOTOS : TRUE

@0
    name : B0
    Define : dm_incohMix_B0 0.507e12, dgog 0.000, qoverp_incohMix_B0 1.00, argqoverp -0.775
    evtGenModel: SSD_CP dm_incohMix_B0 dgog qoverp_incohMix_B0 argqoverp 1.0 0.0 1.0 0.0
@1
    name : KS0
    evtGenModel: PHSP
@2
    name : JPsi
    evtGenModel: VLL
@3
    name : Pip
    smear : LHCbGeneric
@4
    name : Pim
    smear : LHCbGeneric
@5
    name : mup
    smear : LHCbGeneric
@6
    name : mum
    smear : LHCbGeneric

[dlanci]

Any chance this can be reviewed? @michalkreps @chrisburr (not sure I'm including all the relevant project maintainers)

[michalkreps]

I will have a look, but do not have time this week and possibly also next week is tricky.

[dlanci]

No problem, that's not urgent. I am also available for a chat in case it can save some effort since it's a lot of changes. Also EvtGen's treatment of oscillation parameters is not always uniform, (see e.g. https://gitlab.cern.ch/evtgen/evtgen/-/work_items/34) so I had to adapt to it using the least ugly hack.