CPAlgorithms: allow for event selection at particle-level

Some users need to run the event selection algorithms using so-called "particle-level" objects (TruthElectrons, TruthMuons, etc.). This requires the following changes to the event selectors:

• make the MET term a configurable option: at reco-level it's most likely "Final" (new default), at truth-level it's most likely "NonInt"
• replace xAOD::ElectronContainer etc. by xAOD::IParticleContainer to be able to pass either reco- or truth-level containers
• in a couple of cases, event selectors need to access charge(), which is not available for xAOD::IParticle. There I simply added dedicated xAOD::TruthParticleContainer handles, and I populate the correct type of handle based on a switch in the python config.