Tag to be used with the 21.X branches of atlas/athena.
Updates in this tag:
- Updated AthAnalysisExternals to use LCG_96b;
* Removed the build of Eigen, FastJet and FastJetContrib from the project as now "new enough" versions are coming from LCG_96b;
- Updated all externals built by AnalysisBaseExternals to be in sync with the versions available in LCG_96b.