Function flatten

3rd/nlohmann_json/include/nlohmann/json.hpp:8343–8348

! @brief return flattened JSON value The function creates a JSON object whose keys are JSON pointers (see [RFC 6901](https://tools.ietf.org/html/rfc6901)) and whose values are all primitive. The original JSON value can be restored using the @ref unflatten() function. @return an object that maps JSON pointers to primitive values @note Empty objects and arrays are flat

    @since version 2.0.0
    */
    basic_json flatten() const
    {
        basic_json result(value_t::object);
        json_pointer::flatten("", *this, result);
        return result;
    }

    /*!
    @brief unflatten a previously flattened JSON value