HepMC::uniqueID should return id rather than barcode for HepMC3.

Preparing the ground for !67602 (merged) in cases where a barcode is being used as a unique ID for a truth particle only then it should be possible to use the id value interchangeably as this is also unique for each truth particle within a generated/simulated event. (Pile-up truth requires the truth event number in additions to uniquely link to a particle.)

Adding placeholder id() methods for xAOD::TruthParticle and HepMcParticleLink which for now just return the result of barcode(). !67602 (merged) will update this to be the GenParticle::id() for HepMcParticleLink and eventually !67195 (closed) will do this for xAOD::Truth.