建立工作空间：名字可以任意取，但里面必须有src文件夹
编译：用source把编译完的workspace刷新到环境变量里，这样系统才知道ros可执行文件在哪，才能运行。否则roslaunch找不到工作空间。
package is the most basic element for catkin make.
catkin make will recursively search packages in src folder, so the following organization is also allowed:
