O2Sim move to new ROOT serialization

[ktf]

O2Sim move to new ROOT serialization

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
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[ktf]

@sawenzel this is the remaining part of the migration away from TMessage (and its extra copy). Probably not critical, but should not harm either, if it works. I am opening this because I had the code laying around, but I have no strong opinion, we can also simply do it another time.

[alibuild]

Error while checking build/O2/fullCI for ac86486 at 2024-03-26 18:18:

No log files found

Full log here.

[sawenzel]

tested in a few runs. Seems ok.