Externals Update, master branch (2020.03.16.)

Updated all projects to atlasexternals-2.0.60.

The changes since atlasexternals-2.0.59 are (atlasexternals@2.0.59...2.0.60):

• Updated the version of GeoModelCore and GeoModelIO to 3.1.1 (pinging @rbianchi and @boudreau);
  • Added a package for building GeoModelTools, but kept the package excluded from AthenaExternals for now;
  • Being able to compile GeoModelTools in a standalone setup also required adding a package for building XercesC;
• Updated to Acts 0.18.0 (pinging @pagessin);
• Cherry picked the missing macOS Catalina fixes from the 1.0 branch of atlas/atlasexternals;
• Added multiple updates to CheckerGccPlugins (pinging @ssnyder);
• Taught flake8_atlas to recognise even more ways in which people may set the OutputLevel on components explicitly (pinging @fwinkl);
• Added some fixes to AtlasLCG and VP1LightExternals to make it possible to build that project in "standalone mode" once again;
• Added a fix to atlas_add_test(...), so that it would report failures correctly when post.sh is not available during the build (pinging @fwinkl).