Improved the current sensing and regulation.

-All units of the variables/constants involved in the current
regulation/measurement are now SI (Newton, Ampere, Volt...).
-Made the current constants more clear and easier to check against the
datasheets.
-Improved the shunt resistor calibration.
-Tuned again the position and current PIDs.
-Added a feedforward component to the PID module, and used it for the
current regulation.