discussion: nlohmann's json library is heavily outdated

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Entwine still uses a 3.6.1 copy (dating ~3 years ago) from the original author in https://github.com/connormanning/entwine/blob/master/entwine/third/mjson/json.hpp, while his current version https://github.com/nlohmann/json#cmake reached 3.11 or something.

With the evolution of C++ in the last years, this 3.6.1 version seems to have become deprecated or unsupported, depending on the compiler/build environment, and sometimes compilers don't accept the code of that version anymore.

Please consider todays https://github.com/nlohmann/json#cmake for Entwine. I hope this also helps to make the current builds and merge requests more stable;...