Findnlohmann_json.cmake Addition, master branch (2020.09.21.)

Added Findnlohmann_json.cmake to AtlasLCG. This is needed to set up a correct RPM dependency for this external, when picking it up from the LCG release.

This was an oversight on my part in !722 (merged). 😦 I didn't realise that nothing will set up the installation of the jsonmcpp package after those changed. 😦

This is to fix ATLINFR-3735.