add std::to_string method for json object

src/json.hpp

@@ -15289,6 +15289,18 @@ inline nlohmann::json operator "" _json(const char* s, std::size_t n)
     return nlohmann::json::parse(s, s + n);
 }
 
+/*!
+@brief user-defined to_string function for JSON values
+
+This function implements a user-defined to_string  for JSON objects. 
+
+@param[in] j  a JSON object
+@return a std::string object
+*/
+inline std::string to_string(const nlohmann::json& j) {
+  return j.dump();
+}
+
 /*!
 @brief user-defined string literal for JSON pointer