Finish HH gen information retrieval

[blinkseb]

End of #91 ; There's now:

• B & Bbar from one H decay
• V1 & V2 from the other H decay (only the on-shell boson is present)
• L+, L-, Nu1 & Nu2 from the V decays
• X for resonant samples.

Both the index & the P4 is saved, before & after FSR. Index == -1 means not found.

[OlivierBondu]

perfect, many thanks !