EventInfo (version 2) Update, 21.0 branch (2020.04.10.)

This is meant to fix the issue discussed in ATR-21094.

Some time ago (with !29934 (merged)) xAOD::EventInfo went through schema evolution in the master branch with xAOD::EventAuxInfo_v2. This unfortunately stopped the "legacy trigger simulation jobs" from being able to read RDO files produced in master with 21.0.

This MR does a few things:

• It introduces the xAOD::EventAuxInfo_v2 class for 21.0.
  • The class introduced here is not exactly the same as the class in master. That's because some of the thread safety improvements present in master are not available (and not necessary) in 21.0. But the member variables of it are the same, and that's what matters for ROOT I/O.
• It creates the xAODEventAuxInfoCnv_v2 converter, which converts xAOD::EventAuxInfo_v2 objects from input files to the "latest" xAOD::EventAuxInfo type, which in 21.0 remains xAOD::EventAuxInfo_v1.
  • In this sense this converter is also not at all the same as what was introduced for master with !29934 (merged).
• Backported a small fix from @ssnyder for SG::copyAuxStoreThinned(...).
  • Since this is the first T/P converter in 21.0 for a non-container xAOD type, the logic issue in this branch did not hurt us before.

I would've also liked to introduce a unit test for the converter, as it is done in master. But unfortunately there's no reference AOD available at the moment with xAOD::EventAuxInfo_v2 yet. We should follow up with @ssnyder after Easter about putting one of those on CVMFS.

Note that this will also have to be cherry-picked into some of the other 21.X branches as well, but I wasn't sure which label I should set for that...