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Keeping all children of decays under all circumstances

Zach Marshall requested to merge zmarshal/athena:21.0_SaveAllLLPDecays into 21.0

In our current default truth strategies (MC15aPlus) we keep only those children of interactions that pass our cuts. Under normal circumstances, for things like hadronic interactions, this makes a lot of sense -- these interactions aren't expected to appear to conserve momentum (since in any case the target is not quite at rest, and there's break-up energy). For decays, that's not quite true. It'd be a big help if for decays (particularly those of LLPs), all the decay products were kept. It's expected that decays are actually a quite rare process, so this should add only a tiny amount of additional space to the truth record.

This solution is one of many that I can imagine given the current code, but it was the simplest (defined as requiring me to write the fewest lines of new code). I think it can be done in more generic fashion with options and flags passing around, but this is rather more delicate and should probably be discussed before implementation. In any case, I think this ought to work.

@jchapman , please have a look and let me know what you think.

@coccaro , @kpachal , @anwang , please have a look. Volunteers to test this would also be very welcome!

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