Revisit speedRacerDrive() code for 2020 robot to give better control mapping.

[coach-streeter]

CoachStreeter isn't completely satisfied with our current drive code. In particular, the mapping function from joystick inputs to wheel velocities has some oddities in it that make driving the robot more difficult than it should need to be. Should add documentation / explanation of how the drive code is supposed to work, hypothesize potential improvements, implement and try out those improvements, and basically try to increase the drivability of the robot for in the future.