New TS digi+rechit pipeline doesn't propagate the fraction of hit energy deposited by beam electrons

[bryngemark]

When we designed the first TS digi (in effect, just a sim hit reconstruction layer), we introduced a variable beamEfrac to keep track of which fraction of a hit's energy was actually coming from beam electrons. This number is then propagated through clustering and tracking, and is a handy method for, at very little extra cost in terms of memory, assess the quality/purity of the tracks found, and to investigate properties of fake tracks.

This metric was dropped in the RecHitProducer, following the logic that this will never ever be an actual reconstructed observable. Fair enough.

I do think it's still important to be able to assess performance of our reconstruction even if it has an improved level of realism (digi with e.g. finite time resolution), and would not have to revert to the old "digi" which will not give the same answer.

Proposed solution To implement keeping this one simulation-level info float attached to our rechit collection.

Alternative solutions considered To make a new truth info collection that can be used, by truth-reco association. I think this will be disadvantageous to our memory/disk space footprint.