Simple vs Advanced Motor/Gearbox Physics

[zhiquanyeo]

Refactor the robot drivetrain classes (SimMotor, SimRobotWheel, SimRobotDrivetrain) to support simple or advanced physics. This is an enhancement to #76

Simple physics:

• The way it is set up now. Each SimMotor takes in a maximum output force, and the drivetrain distributes that force among the wheels

Advanced physics:

• SimMotor takes in Free speed, stall torque, etc
• Motor Groups act like gearboxes where reductions can be specified
• Torque produced by wheels is calculated and converted into pushing force
• Potential for SimMotor to calculate current draw

A first step will be to refactor the code to allow a swappable "drivetrain physics" module, and write a "SimpleDrivetrainPhysics" module that behaves the same way existing code does