Updates for signal samples (primarily SVJ)

The SVJ-oriented changes are:

• allow disabling PDF weight normalization
• store SVJ GenParticle chains (with a hack to keep first copies of HV quarks)
• prepare for upcoming SVJ signal scans

Some of these changes are not quite backward compatible for V17:

• new SignalParameters branch generalized to handle any type of signal scan (SusyMotherMass and SusyLSPMass branches kept for compatibility, plan to remove them after V17 finishes)
• GenParticles_ParentIdx branch filled, when possible, for "individual" particles kept in the minimal scheme
• GenParticle chain added to minimal scheme for SUSY signals

but the impact is minor, so we will go ahead.

attn @aperloff for GenParticleProducer changes