Update ROS wrapping to use ROS Velodyne driver 1.6

ROS Noetic packages on Ubuntu 20 use ROS Velodyne driver v1.6. This new version is not backward compatible.

Especially, this new driver adds a point-wise timestamp value, and changes the PCL point definition namespace, leading to API break.

This MR updates ROS wrapping in order to use new Velodyne driver 1.6.

If you're running on Ubuntu 20/ROS Noetic, you can install the new Velodyne driver using the command sudo apt install ros-noetic-velodyne ros-noetic-velodyne-pcl. If running on previous versions of ROS/Ubuntu (Melodic/18 and below), you need to compile this driver from source : just clone the git repo in your catkin worskpace, it will be automatically compiled.

@julia.sanchez @nick.laurenson PTAL