add support for spherical wheels

[frantisekbrabec]

This adds ability to retrieve wheel radius from spherical wheels (not just cylindrical). This code was shamelessly taken out of https://github.com/ros-controls/ros_controllers/blob/noetic-devel/diff_drive_controller/src/diff_drive_controller.cpp

[bmagyar]

don't worry, pretty much all code in this repo has been shamelessly taken from that controller ;)