TruthUtils/AtlasPID: Re-order helper function definitions

Currently there is a circular dependency in the definition of the isValid helper function for pdg_id values passing isHiddenValley and isTechnicolor. Breaking the dependency requires re-ordering of the AtlasPID.h header. It is cleaner to do this re-ordering as a separate MR. I also used the opportunity to improve the comments on some of the helper functions.