Add getters and labelling decorations

Adds some getters to the FTAG discriminants package, also ordered existing getters for more logical groupings.

• abs_deta is for training with absolute jet eta
• a few other basic quantities defined here

Future work: splitting off generic getters that can also be fetched for IParticle objects, to avoid redefinition here.

I also added the following to ParticleJetTools labelling

• HadronConeExclTruthLabelPtScaled which can be used as a jet energy regression target
• HadronConeExclTruthLabelDR, the dR(labelling particle, jet)
• HadronConeExclTruthLabelPdgId, the pdgID of the labelling particle (suggested by @cpollard)

Tagging @dguest