23 linkage between low level and high level reco objects

[mt82]

Summary

This implements the reconstructed particle–lower-level object linkage as described in the architecture sketch:
https://github.com/DUNE/duneanaobj/wiki/Reco-Particle--%E2%80%90--Lower-object-architecture-sketch

Changes

• Added two identifier classes:
  • SRRecoParticleID to identify a reconstructed particle
  • SRRecoBaseID to identify a reconstructed low-level object
• Introduced a common base class for reconstructed objects: SRRecoObjBase.
  All reconstructed object classes will inherit from this base.
• Extended:
  • SRRecoParticle to hold an SRRecoParticleID
  • SRRecoObjBase to hold an SRRecoBaseID
    This establishes a bidirectional link between reconstructed particles and low-level reconstructed objects.
• Added helper functions to retrieve:
  • SRRecoParticle instances from an SRRecoParticleID
  • SRRecoObjBase instances from an SRRecoBaseID

[chenel]

This is amazing. Thank you so much for following it through!

My only real comment that you'll see below is I think we want a state-checking method for the ...ID classes that encapsulates the logic checking whether they contain a valid value. I've implemented it in suggestions as an operator bool() overload, but we can discuss if IsValid() (or similar) would be better.

In order to actually merge this, I think we would want to have a paired ND_CAFMaker PR where we actually fill these fields in, so that we can be at least somewhat sure it does something sensible. Up to you whether you want to take a stab at that, or try to recruit someone else 😄

[wyjang-uta]

Thank you for the contribution @mt82 and a review, @chenel !

@mt82 Please update the commits based on the reviewer's feedback. Once the requested improvements are complete, we will proceed with merging this for the next release.