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Ros node启动与关闭
1. ros运行单位：
Ros程序运行的单位是ros node。
2. ros 节点的启动：
（1）初始化ros节点：通过调用ros::init()接口实现；可以通过参数指定默认节点名字，之所以是默认，是因为在命令行执行程序时，是可以修改节点名字的。
（2）启动ros节点：通过调用ros::NodeHnadle创建NodeHnadle实例nh实现，第一次创建实例时，会调用ros::start()，此时启动本节点。
3. ros节点关闭：
（1）通过调用ros::shutdown()接口，关闭ros节点，此接口会关闭订阅话题、发布话题、请求服务以及提供服务。
（2）在最后一个ros::NodeHnadle对象销毁时，会自动调用ros::shutdown()，以此关闭ros节点。
4. 检测ros节点是否已经关闭：
（1）ros::ok()接口：返回false，说明节点已经关闭。
（2）ros::isShuttingDown()接口：返回true，说明ros::shutdown()被调用，但是节点比一定结束。
注：所以一般使用ros::ok()判断，但是在不会结束的节点，例如“ prolonged service
callback”中，则只能使用ros::isShuttingDown()，因为此节点不会关闭，疑惑这是什么节点？
5. 中断：
（1）在ros::init()选项中，有一选项ros::init_options::NoSigintHandler，决定是否安装默认SIGINT处理函数，若不带此参数，则安装，程序在收到SIGINT信号（ctrl+c）时，ros::ok()返回false，节点可以走关闭流程。若带此参数，需要自己安装SIGINT处理函数，以保证节点可以结束。
