Tag to be used with the 21.X branches of atlas/athena. Updates in this tag: - Fixed up the CMake version requirements in the code a bit; - Added a find-module for the C++ Guidelines Support Library; - Fixed the environment setup for matplotlib from LCG_94; - Upgraded KLFitter to version 1.2.1; - Added fixes for building ROOT 6.14/04 on bare-bones Ubuntu systems.