Updated all "standalone externals" to the versions provided by LCG_98python3. Most importantly I switched to ROOT 6.22/00 and Python 3.7.6 for the AnalysisBase build.
Removed the FFTW package from the repository, setting up AthenaExternals to rather pick it up from LCG;
I also had to update a number of packages in this repository to deal with these updates. Most of them were trivial issues arising from the external version updates for analysis packages, and from previous build configuration mistakes that were highlighted by the change in where FFTW, nlohmann_json and GoogleTest would be picked up for the Athena project.
I had to update RootUtils, since for some unknown reason in a standalone build I had to link it explicitly against libcppyy. (I don't know why, since the libROOTTPython.so dependency should automatically pull in the libcppyy3_7.so dependency as well... ) And while at it, I cleaned up the CMake configuration of that package a bit.
Finally, I did some housekeeping in the projects. Synchronising the version numbers of all of them, and making sure that AthDataQuality would really use LCG_98python3. @wlampl, unfortunately I missed this, that !35630 (merged) was doing this update incorrectly. (But since we don't build AthDataQuality in the master branch in either the CI or the nightly, this did not show up as an error.)
