Powercube chain offset fix

We had the following issue with the tray: It was disconnected from the power supply for a longer time and lost its internal calibration. Therefore, we wanted to add the calibration offset in the calibration.urdf.xacro of COB3-1. Then however, the tray was not moving anymore, because the limits were not calculate correctly.

This pull request adds two helper functions, which allow the conversion between joint encoder values and ROS joint values, which are semantically easier to understand than adding or subtracting offsets. For our robot, this solves the issue which we had with the tray. The torso (without calibration offsets, but using the same code) is still working, though.