使用nlohmann进行修改json文件

参考github问题：

https://github.com/nlohmann/json/issues/1475

    std::ifstream in("/LearnJSON/test2.json");
    json file = json::parse(in);

    json j;
    j = file;
    j["unhappy"] = false;

    std::ofstream out("/LearnJSON/test2.json");
    j["pi"] = 3.14159;
    out << std::setw(4) << j << std::endl;
    float test = j["pi"];
    std::cout << "New variable is: " << test << std::endl;