[SPI-2042] New layer LCG_101_ATLAS_8
New layer LCG_101_ATLAS_8:
- sherpa and sherpa-openmpi is built against HepMC2 only;
- added magraph5amc 3.3.1.atlas1 (based on 3.3.1) with all ATLAS patches.
- add coverage and pytest to ATLAS top_packages
- coverage is update to 6.2