extend genMatchable to also use prunedGenParticles

[btcardwell]

At the moment, genMatchable uses the packedGenParticles collection, which does not have the correct vertex information for electrons and muons. To get around this, we currently use separate matching logic in OSUElectronProducer and OSUMuonProducer, but it would be better to extend genMatchable to work with either MC particle collection.

See #162 for further discussion.