xAOD (Standalone) Reading Updates, master branch (2020.03.20.)

Before starting with the really difficult bit (merging the changes that happened in Control/xAODRootAccess independently in 21.2 and master), I wanted to start with something a bit less controversial first. 😉

The MR does the following things:

• It updates the AsgExternal/Asg_Test package (what a name...) to point to the files on CVMFS that we use at the moment in all of the unit tests in the 21.2 branch;
  • This should also help with the failures observed in !31327 (merged);
• Made some inconsequential cleanup in Control/xAODRootAccessInterfaces, which are just meant to make the code a bit cleaner/prettier;
• Added the Control/xAODDataSource package from 21.2, almost as-is. I just had to make some small adjustments in its CMakeLists.txt file to make it compile as part of both Athena and AnalysisBase.