Cherry-pick "Merge branch 'Findnlohmann-master-20200921' into 'master'" into 1.0
Findnlohmann_json.cmake Addition, master branch (2020.09.21.)
(Closes ATLINFR-3735)
See merge request !727 (merged)
(cherry picked from commit e4e3f481)
- d107dbfb Added Findnlohmann_json.cmake to AtlasLCG.