Tag to be used with the 21.X branches of atlas/athena. Updates in this tag: - Made AnalysisBaseExternals functional on the PPC64 platform. - Added some dictionary improvements for detecting upstream changes more reliably that should trigger a dictionary re-generation. Also made the dictionary generation more reliable on MacOS Mojave. - Updated AnalysisBaseExternals and AthAnalysisExternals to LCG_94a. This meant updating the version of ROOT and Davix built for AnalysisBaseExternals.