Tag to be used with the master branch of atlas/athena. Updates since 2.0.15: - Added FindSuperchic.cmake and updated FindHerwig3.cmake to work with newer Herwig versions; - Updated to CLHEP 2.4.0.1; - Updated to Geant4 10.4.2.1; - Added GeoModel as an external to AthenaExternals and AthSimulationExternals; - Added a speedup to calling find_package(LCG ...) in user projects.