Add Robosense RSLidar conversion node, and improve approximate point-wise timestamp computation

• Add conversion node to use RS16 data (this may also work with other RS Lidars, but some things would need to be checked)
• Improve approximate point-wise timestamps computation : the computed time field is now the time offset to add to header.stamp to get "real" point-wise time. This includes adding the timestamp_first_packet for Velodyne conversion node.
• Change lidar_frequency option to rpm to mimic ROS drivers parameters.

@julia.sanchez @nick.laurenson @melanie.carriere PTAL