Externals Update, master branch (2020.12.09.)

Updated all projects to atlasexternals-2.0.92. The changes wrt. atlasexternals-2.0.91 are the following (atlasexternals@2.0.91...2.0.92):

• Modernised the CMake configuration files exported/installed by atlas_project(...);
• Added Findnumpy.cmake and Findmatplotlib.cmake to AtlasLCG (pinging @amete);
• Fixed the (standalone) build of XRootD on macOS;
• Added CLHEP and yampl to AthAnalysisExternals.

The addition of Findnumpy.cmake and Findmatplotlib.cmake came up in !38852 (merged).

The tweaks to AthAnalysisExternals are necessary for ATLASG-1552 and !39015 (merged).

But the dangerous part of this update is the change made in AtlasCMake (atlasexternals!780 (merged)). It, along with all the other updates, does not require a full-build. However if I made a mistake with it, it has the chance of seriously breaking the nightlies. As discussed with @fwinkl in atlasexternals!780 (merged), I would propose to only merge this in on Friday. (Assuming that the CI will be all happy...) So that any issues with the nightly could be sorted out during the weekend.