Changed output topic to sensor_msgs/ChannelFloat32 instead of std_msgs/Float32

The distance read from the ultrasonic sensor is now output as ChannelFloat32 from the sensor_msgs package instead of just using std_msgs/Float32. This has two reasons:

1) The meaning of the data gets clearer (a "distance" field name is attached)
2) The overall purpose of the sensor_msgs package is to support sensory output