Externals Update, master branch (2021.02.16.)

Updated all projects to atlasexternals-2.0.100. The changes wrt. atlasexternals-2.0.99 are (atlasexternals@2.0.99...2.0.100):

• Updated FindGeant4.cmake to be compatible with Geant4 10.7.X (thanks to @bmorgan);
• Updated prmon to version 2.2.0 (thanks to @amete);
• Added FindApfel.cmake to AtlasLCG (thanks to @ewelina);
• Fixed a mistake in how flake8 tests would be run by atlas_install_python_modules(...) (thanks to @fwinkl).

Frank, I didn't see any problems with the python modules in my local test build. So 🤞.

None of these changes require a full-build, so let's just do an incremental one...