Remove ReconstructedParticle::PDG member?

[c-dilks]

Is your feature request related to a problem? Please describe.

After some discussion in recent Software & Computing meetings, there may be some interest in removing the scalar member PDG from ReconstructedParticle, which in EICrecon is presently used for true PDG. Some points from these discussions:

• MCRecoParticleAssociation::getSim::getPDG can be used to provide true PDG instead
  • not every out ReconstructedParticle object will be found in a MCRecoParticleAssociation, do those have a true PDG?
  • what should we do about members which may depend on PDG, such as ReconstructedParticle::mass? Or are these members filled without knowledge of the true PDG?
• Removing member PDG brings us closer to EDM4hep model; it would be nice to switch to edm4hep::ReconstructedParticle, however, relations such as vertex, track, and cluster are to the EDM4eic objects, so this does not seem practical at this time
• Removing member PDG will break:
  • Juggler
  • Any EICrecon reconstruction algorithms which use it
  • User analysis code
• ReconstructedParticle::getParticleIDUsed and ReconstructedParticle::getParticleIDs will be used for PID objects, which in turn contain the PDG value(s) from PID

Let's use this Github issue to continue the discussion and eventually converge on a path forward for the future.
@sly2j @Chao1009 @wdconinc @mdiefent