Externals Update and AthGeneration Project Creation, master branch (2020.02.19.)

The MR updates all projects to atlasexternals-2.0.57. The list of changes are (atlasexternals@2.0.56...2.0.57):

• atlas_add_test(...) now always runs the post-processing script (if one is defined), even if the test executable failed (thanks to @fwinkl);
• Updates to CheckerGccPlugins (thanks to @ssnyder);
• atlas_generate_cliddb(...) no longer generates/uses a dummy_clid.db file (thanks to @fwinkl);
• atlas_project(...) now only calls FindDoxygen.cmake if absolutely necessary (thanks to @fwinkl);
• Fixed the patching of Acts when building the project with Clang (thanks to @ssnyder);
• Added pyAMI to the list of modules built by the PyModules package (thanks to @fwinkl);
• Added the AthGenerationExternals project to the master branch;
  • Updated the build configurations of FastJet(Contrib) and HepMCAnalysis to accommodate the "new" project.

For the last update (atlasexternals!612 (merged)) I had to make some changes in this MR as well, to start off the AthGeneration developments in the master branch.

Unfortunately a lot of code updates that were made in 21.6 are not available in master yet. I updated the xAODJet package to behave as it should (while also cleaning the code a bit), but once I realised how much work is necessary for MCTruthClassifier, I decided to give up. 😦 At this setup the build of AthGeneration at least "starts up". But then runs into a number of compilation issues.

But those I would prefer to leave to PMG to sort out. In future MRs. Possibly after an AthGeneration-master-x86_64-centos7-gcc8-opt nightly was already set up. (@aundrus, you'll probably get a request for this soon...)

Tagging @averbyts and @ewelina on this. In the end I decided that it will be easiest if we just merge this MR. Should make it easier for you guys to build on top of this.