Add truth counters to xAODParticleEvent's CompositeParticle

The CompositeParticle class holds multiple particles. It has useful methods like std::size_t nElectrons() const; to count the number of a given particle species.

When running over truth derivations, these methods return 0, because all particles are xAOD::TruthParticles rather than xAOD::Electron etc. To be able to use similar methods for a truth analysis, this MR adds methods of the form std::size_t nTruthElectrons() const;. They count the number of xAOD::TruthParticles, whose pdgId matches the corresponding particle.

The additional methods are:

nTruthPhotons()
nTruthElectrons()
nTruthMuons()
nTruthTaus()
nOtherTruthPhotons()
nOtherTruthElectrons()
nOtherTruthMuons()
nOtherTruthTaus()

Note that no additional methods are needed for jets, because truth jets are xAOD::Jet objects.