Add enum/bitmask field for `ReconstructedParticle` to indicate which subsystems it came from

[veprbl]

While ReconstructedParticles collection (not to be confused with the type of the same name) is meant to contain all particles detected in the event, but user analyses might want to select particles from a specific detector (i.e. specific information available).

There are two proposals for this. We could make an enum to describe certain regions of the detector (e.g. fb, backward, central, forward, ff) or we could make a bitmask (like MCParticle::generatorStatus in EDM4hep) and assign bits to each subsystem. Whenever information is added from a certain subsystem, its bit is set to 1.

cc eic/EICrecon#1946 (comment)

[ruse-traveler]

I've been thinking on this, and I like the idea of the bitfield! I feel that would help us keep all of the definitions in one place...

I'm curious about what would this look like in implementation. Would we define a handful of ints to specify which subsystems were included? Maybe something like:

edm4eic::ReconstructedParticle:
...
members:
  - int64_t subsystemsUsed
...
mutable extra code:
  includes: "#include edm4hep/bit_utils.h"
  declaration: |
    // the subsystems' bit positions
   static const int64_t FEMC = 0;
   ...   
   bool usedSubsystem(const int64_t bitsys) const { return edm4hep::utils::checkBit(getSubsystemsUsed(), FEMC); }

[veprbl]

Yes, we should adopt code from similar implementations in EDM4hep.

https://github.com/key4hep/EDM4hep/blob/3d1abee61435162661650cab4e10f5395047d46b/edm4hep.yaml#L259-L276

[ruse-traveler]

Definitely! What are your thoughts, @ajentsch @rachel-m @simonge ?