Build appraoch for nlohmann josn & validator

[dvstans]

Recently rebuilding DataFed on a new machine, I ran into the issue of the nlohmann josn and schema validator libraries not having released linux packages (they are GitHub open source projects). The process for installing these is also not documented. We should either document this, integrate these libraries directly into DataFed, or find better supported alternatives.

Note that the nlohmann json library is a dependency for the schema validator. If we move schema validation to the web api layer, we could remove both of these dependencies for the core server.

[JoshuaSBrown]

I only have a comment on this, the devel branch now has a script for installing all dependencies that will automatically pull the repos down and build them.

I do not have a strong opinion on this yet, as I have not used these third-party packages much directly. I am always up for reducing dependencies if possible but I leave it up to your judgment as to how to proceed with this.