xAODDataSource, 21.2 branch (2019.11.22.)

After keeping this project on the back burner for a very long time, I now spent a few hours to make the code that I kept in akraszna/xAODDataSource, a part of the analysis release. As is written in the code's comments, I wrote the code based on the work that Umesh Worlikar did a while ago. (I didn't find his user ID in GitLab anymore, I think he left ATLAS by now...😦)

The code itself is not super trivial, so probably an expert review will be warranted. (@krumnack 😉)

The main/only user interface of the code are the 2 overloads of the xAOD::MakeDataFrame(...) function. People should really only use that, they shouldn't have to interact with any other implementation detail inside the package.

More documentation will have to be written later on, for now the unit tests of the package could serve as examples of what the code allows the users to do.

Finally, I've only added the package to the AnalysisBase and AnalysisTop projects for now. Since I've only done tests on top of those projects for now. In the past I did test the code with AthAnalysis as well (see for instance: https://gitlab.cern.ch/akraszna/xAODDataSource/pipelines/574135), so maybe I'm too careful here. 🤔 We may just want to keep this MR open until I'm back in front of a CentOS 7 machine, where I'll be able to make some additional tests before adding the package to AthAnalysis as well...