Manual sweep of SystematicsSvc improvements from master
This is a manual sweep of many MRs from master. No fixes are done (only syntax changes). The changes got a bit messy in the process, so marking to be squashed.
Also including atlas/Athena!39398 to ease sweeping in the future.
@krumnack, please try to have a quick look that I did not mess-up something.
I'll prepare some fixes in parallel for transparency. We should coordinate that all connected MRs would go in before a new release will be built.