Externals Update, 21.0 branch (2020.09.22.)

Updated all projects to atlasexternals-1.0.48.4. The list of changes wrt. atlasexternals-1.0.48.1 (which Athena was using, but every other project was still using atlasexternals-1.0.48...) is the following (atlasexternals@1.0.48.1...1.0.48.4):

• Enabled the build of HDF5 as part of AthenaExternals (should not affect 21.0);
• Backported the fix for building Simage on CentOS 7 from the 1.0 branch (fixing ATLINFR-3737);
• Updated FindUUID.cmake to set up a correct RPM dependency for the uuid LCG package on CentOS 7;
• Updated the build of yampl to use libuuid from LCG when it is coming from there (fixing ATLINFR-3736).

Pinging @rbianchi.