Copter: Add voltage scaling to PIDs

I disagree that the PIDs don't care about noise in the scaler: if there's any error being input to the PID, any noise in the scaler will be directly passed through to the output of the PID since it's a linear system and this change multiplies the error by the scaler before passing it to the PID. So this change will increase the noise in the motor outputs, which will lead to unnecessary vibration depending on how noisy the voltage pin input is. It should be pretty easy to run it through a LPF, no?