Externals Update, 21.6 branch (2020.06.04.)

This MR updates the 21.6 branch to atlasexternals-1.0.65, and makes a few other improvements along the way.

The full list of changes wrt. atlasexternals-1.0.60 is (atlasexternals@1.0.60...1.0.65):

• Updated AthAnalysisExternals to use LCG_96b (not relevant for AthGeneration);
• Updated all externals built by AnalysisBaseExternals to be in sync with the versions available in LCG_96b (not relevant for AthGeneration);
• Bumped the version of MCUtils to 1.3.4 in AnalysisBaseExternals and AthAnalysisExternals (not relevant for AthGeneration);
• Removed the unnecessary -c argument from the CINT dictionary generation calls (no change expected for AthGeneration because of it);
• Enabled the build of Minuit2 for/with ROOT 6.18/04 (not relevant for AthGeneration);
• Introduced the OnnxRuntime external, and added its build to the AthenaExternals, AthAnalysisExternals, AthDerivationExternals and AnalysisBaseExternals projects (not relevant for AthGeneration);
• Taught FindSherpa.cmake to add the location of the Sherpa and Sherpa-config executables to the PATH environment variable (the main purpose of this MR!).

While working on the branch I discovered some other imperfections as well.

• I really didn't like the usage of the HW3_VER_IS_72 cmake variable. I instead modified the code to rely on HERWIG3_LCGVERSION directly.
• There was a typo in setting up with version of Crmc the project should build against. Which resulted in picking up a different version than was intended.

And finally (and this is why I started all of this in the first place), with FindSherpa.cmake now adding Sherpa-config to $PATH in the runtime environment, removed the hardcoding of the location of this script into the C++ code. Which should make the generator software a bit more relocatable. 😉

Ping @ewelina, @cgutscho.