Add TruthParticleWriter

Add the ability to write out truth particle information alongside jet and track information. The main use case is to write out information about b and c hadrons in the jet, but we might also want to write out lepton or other information later on.

• JetTruthDecorator takes a jet and selects some truth particles to ElementLink to the jet
• TruthSelector handles the truth selection, and the accessing of the linked particles
• TruthWriter writes some information about the selected truth particles to h5.

At the moment we write out for hadrons / leptons

• pt
• eta
• phi
• dr
• Lxy
• pdgId
• barcode

At some point we should associate the hadrons to the truth vertices as @cschiavi suggests. But that will have to wait for another MR as this one is already getting large.

See discussion at #30 (closed).

Quick validation plot for the hadrons: