diff_drive_controller code tidy-up + many tests are missing from ROS1

Review the code and feature list of diff_drive_controller from ROS1 and write up a comparison with the current version in ROS2, don't forget about testing either. Please make sure to highlight messy or inefficient parts in the current ROS2 implementation, there should be a few of those still...

https://github.com/ros-controls/ros_controllers/tree/noetic-devel/diff_drive_controller https://wiki.ros.org/diff_drive_controller

The ticket then should be extended with either a list of TODOs or a list of additional tickets to address the missing parts.