Rivet: Particle pair classifiers

| Rivet 3.1.9

Functions
Particle pair classifiers

## Functions
bool | Rivet::PID::isSameSign (PdgId a, PdgId b)
bool | Rivet::PID::isOppSign (PdgId a, PdgId b)
bool | Rivet::PID::isSameFlav (PdgId a, PdgId b)
bool | Rivet::PID::isOppFlav (PdgId a, PdgId b)
bool | Rivet::PID::isOSSF (PdgId a, PdgId b)
bool | Rivet::PID::isSSSF (PdgId a, PdgId b)
bool | Rivet::PID::isOSOF (PdgId a, PdgId b)
bool | Rivet::PID::isSSOF (PdgId a, PdgId b)

## Detailed Description

Todo:
  Make versions that work on PdgIdPair?