Synchronizing with 26.2 ms communication

[Alan Anderson]

First, we perform the PID calculations based on a timer every 26 ms instead of the 20 ms we had been using.

Second, and more important, we switched from trying to control the wheel velocity and went to a position-based PID scheme instead. Now the control parameter is the desired robot location (and orientation) instead of the desired speed (and turn rate). The PID constants take care of making the velocity what it needs to be in order to get to the right location without overshooting. For manual driving, the joystick input constantly sets the desired position in front or in back of the current location, and when that point is set farther away, the robot travels faster.