consider adding JSON Schema validation in SLiM

[bhaller]

This would considerably simplify development on SLiM's schemas, since right now validation testing happens in python. It looks like this is a library we could use to do it:

https://github.com/pboettch/json-schema-validator/tree/main

It is compatible with nlohmann, and doesn't say anything about requiring C++ versions beyond C++11. It is not header-only, but it looks like maybe there would be only a single .cpp file.