Externals Update, master branch (2020.03.30.)

Updated all projects to atlasexternals-2.0.61.

Changes wrt. atlasexternals-2.0.60 are (atlasexternals@2.0.60...2.0.61):

• Added new checks and warnings to CheckerGccPlugins (cc. @ssnyder);
• Updated the versions of GeoModelCore, GeoModelIO and GeoModelTools (cc. @rbianchi, @boudreau);
  • Re-enabled the build of GeoModelTools in AthenaExternals at the same time.

The GeoModel updates are meant to fix the issue described in ATLASDD-35 and ATLASSIM-4477. They necessitated some changes in GeoSpecialShapes and VP1Utils, as explained in the discussion of the MR.