Add sinusoidal disturbance command.

Set a sinusoidal disturbance to add to the control torque, which takes
two args, the amplitude and the frequency. Amplitude is in N-m,
frequency is in Hz. Disturbance will be enabled after the bike state
norm is less than 0.5.
Enable disturbance after reaching speed setpoint.
Specify disturbance amplitude in centi Newton-meters.