ROS node to save LIDAR and RC inputs for machine learning training data

[5yler]

This needs to be updated because the values on arduino/radio are invalid. The gigatron_hardware/Radio.msg definition incorrectly had the speed command fields as uint8 instead of int16, so all throttle radio commands from rosbags before the gigatron_msgs refactoring are unusable - though we can use the actual velocity values of the car instead.

[5yler]

Using the usec_left values on arduino/motors instead of velocity on the state topic because they are equivalent to the radio commands received at the same timestep, as converted by the formula:

mot_msg.usec_left = 1500 + (2 * radio_msg.speed_left)