MCUtils Update, 1.0 branch (2020.04.14.)

This is meant as a replacement for !651 (closed). (@zmarshal, next time please give me [and all people with administration privileges] modification rights to your branches that you create merge requests out of...)

On top of Zach's update of MCUtils for AnalysisBase, I also added the "build" of that package to AthAnalysisExternals. So the same version would be used in AthAnalysis as well.

At the same time: Let's not forget about adjusting https://gitlab.cern.ch/atlas/athena/-/blob/21.2/Projects/AthDerivation/externals/MCUtils.cmake, and removing https://gitlab.cern.ch/atlas/athena/-/blob/21.2/Projects/AthAnalysis/externals/MCUtils.cmake when we collect this update into the nightly! (It's a bit weird that LCG_88 [used by AthDerivation] does have this version of MCUtils as part of it, while LCG_96b, which is used by AthAnalysis, does not.)