Velocity Motor Controller

[varun0630]

Velocity motor controller would be an additional layer in our stack. We'd convert our existing controllers to output voltages instead of velocities, then hand those voltages to our velocity motor controller that converts the velocity targets to voltages and commands the motors. The thought is that this could significantly improve the performance of the arc pid controller (see #118). Additionally, if successful, this could be applied to other controllers and future controllers.

The first thing we should check is if the integrated velocity controller is suitable for this task before we try to write our own motor controller.

[AndrewLuGit]

Steps to implement:

1. Add velocity command to chassis/ChassisCommand.hpp. We can choose to scale velocity to the range [-100, 100] to mirror the voltage scaling, or we can choose some other unit (rpm, linear speed) instead.
2. Add handler for velocity commands in DiffChassis::execute function in chassis/DiffChassis.hpp. This will involve either calling move_velocity to use the motor's builtin velocity PID, or if that doesn't work well, having our own velocity PID controllers.
3. Modify controllers to return velocity instead of voltage commands, and see how well they work.