More accurate physics for motors

[zhiquanyeo]

The SimMotor currently uses a very simplistic model of a maximum output force, and an input signal. This works fine for demos and low-fidelity simulations, but we could probably do better.

This issue involves investigating and potentially buidling a new Motor class that allows you to set maximum free speed, stall torque etc and take into account the mass of the robot, diameter of the wheels etc, to calculate a more accurate force value for each wheel.

We might want to make this a plugin to the SimRobotDrivetrain so that we can swap physics models if necessary. Additionally, this might open the doors to us simulating current flow through the motor, for more advanced simulations.

[zhiquanyeo]

Superseded by #90