ros:发布里程计信息

里程计消息nav_msgs/Odometry 结构如下

Header header 
string child_frame_id 
geometry_msgs/PoseWithCovariance pose 
geometry_msgs/TwistWithCovariance twist

协方差解释：
As a robot moves around, the uncertainty on its pose in a world reference continues to grow larger and larger. Over time, the covariance would grow without bounds. Therefore it is not useful to publish the covariance on the pose itself, instead the sensor sources publish how the covariance changes over time, i.e. the covariance on the velocity. Note that using observations of the world (e.g. measuring the distance to a known wall) will reduce the uncertainty on the robot pose; this however is localization, not odometry.

来自robot_pose_ekf;
这里说的是，协方差表示的是协方差如何变