Externals Update, master branch (2021.01.22.)

Updated all projects to atlasexternals-2.0.97. The list of changes wrt. atlasexternals-2.0.95 is (atlasexternals@2.0.95...2.0.97):

• Switched to building all GeoModel libraries from the "new" GeoModel mono-repository;
  • I had to include a C++ update from !36896 (closed) to make this work;
• Upgraded CLHEP to version 2.4.1.3;
• Upgraded Geant4 to version 10.6.3.2;
  • I had to make a small update in G4HadIntProcessor.cxx to make this work;
  • Plus I added small changes in a number of C++ source files to prevent compilation warnings with the new G4 version;
• Tweaked the linking options on the aarch64 platform.

@rbianchi, @tsulaia, note that since I kept all of the previous GeoModel find-modules in place, I only had to do this one C++ change to make this MR work. Otherwise I would've had to update O(30) CMakeLists.txt files in this MR as well.

@bmorgan, in the end there was one missing include error that was brought out by the Geant4 version update. I didn't do any runtime tests, but I guess the validations that you guys did, should make sure that the CLHEP/Geant4 updates will be reasonably safe.