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

[bmagyar]

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.

[AugusteBourgois]

I just came across you issue as I was looking for a reason why the update function of the diff_drive_controller started by checking whether the controller was activated or not.
After looking at the previous versions of the code, and in particular cross checking with the controller_manager package, it seems to me that this could be an old artifact from the eloquent version of the controller, as in the foxy and galactic version of the controller_manager, the latter checks the controller's state before updating it.
Could this be, by any chance, one of the "messy and inefficient part" of the current code version you're talking about ?