Linkage between 'low-level' and 'high-level' reco objects

[chenel]

At the moment there's no way to tell which low-level (detector-specific) object(s) a SRRecoParticle was made from. This is often information needed to make further high-level decisions.

I attemped (in branch feature/reco_linkage) a structure where every low-level CAF reco object would inherit from a base class, and the SRRecoParticle would maintain a pointer back to it. Unfortunately that isn't compatible with SRProxy, which only understands POD types (and vectors of them).

I also have considered a structure where SRRecoParticles maintain something like the SRNDLArID (etc.) that gives the relevant indicies back to their reco-obj-of-origin. The trouble with this is that this object-of-origin could be anywhere in the StandardRecord and we'd have to do some very unwieldy thing that can map out all the possible places it could live.