Externals Update, master branch (2020.07.27.)

Updated all projects to atlasexternals-2.0.72. The full list of changes wrt. atlasexternals-2.0.71 is (atlasexternals@2.0.71...2.0.72):

• Fixed a typo in the documentation of CheckerGccPlugins (thanks to @fwinkl);
• Added Findpsutil.cmake to AtlasLCG (thanks to @amete);
• Updated Acts to version v0.29.0, and updated AtlasLCG to allow it to find Eigen's own CMake configuration in the LCG releases (thanks to @pagessin);
• Updated prmon to version 2.0.2 (thanks to @amete);
• Updated LHAPDF to version 6.2.3 for the analysis projects (for Python 3 compatibility);
• Updated all LCG based projects to use LCG_97apython3_ATLAS_1 by default.

At the same time, since the new tag uses LCG_97apython3_ATLAS_1 by default for all LCG based projects, removed the overrides from the build_externals.sh scripts of all projects, which were forcing the usage of LCG_97a_ATLAS_1 by default so far.

Also updated the Athena configuration to use the TDAQ installations compatible with Python 3 instead of Python 2.7, and removed the NumPy environment setup from the project. Since with LCG_97a we don't need that anymore. The latter will fix ATLINFR-3652.

This is meant to conclude ATLINFR-3645 from a "software perspective". @aundrus, once this MR is in, you will be able to remove all explicit settings of the CI for LCG_VERSION_NUMBER, LCG_VERSION_POSTFIX, TDAQ-COMMON_VERSION and TDAQ_VERSION. In the nightlies all "standard" nightlies should lose explicit settings for these variables as well. While the Python 2 nightly should not be touched, it should continue to be built as we agreed on in this comment.

To the release coordinators: I'm omitting the full-build tag on purpose. I don't think any of these changes require a full build. And I'd rather that this CI would reveal if I'm wrong, rather than all subsequent CI-s doing it once this is merged...