diff drive skid steer

This PR is an update from the hydro-devel branch of ros_controllers from PAL Robotics, and it depends on https://github.com/ros-controls/ros_controllers/pull/137. 

It adds support for multiple wheels per side to the diff_drive_controller.

This version of the diff_drive_controller is the one that is currently running on all PAL Robotics platforms and on the Jackal from Clearpath Robotics.

Related PRs:
https://github.com/pal-robotics/ros_controllers/pull/42
https://github.com/ros-controls/ros_controllers/issues/100

__WIP 
I have to fix something on the xacros because currently the tests are failing on them__