Boost.Json issue in multiple compilation units

[rogerpearce]

https://github.com/LLNL/ygm/blob/1ff21d42abd973261c2cd2d67bdc437d3032fa46/include/ygm/utility/boost_json.hpp#L12

Could we not include the /src.hpp file here, and force applications to link agains Boost.Json? The src include file does not work with multiple compilation units.

[steiltre]

This and (hopefully) all issues with multiple compilation units are being taken care of in PR 389.