ST10Controller: Unconditionally stop motor before starting new move

[alexdewar]

The stepper motor controller has this interesting feature whereby every time you send a move command it enqueues it in a buffer, so you can tell it to move to point A then point B. We never use this feature, however: the frontend just sends a stop command immediately before it sends a move command. As with #704 though, this is really a case of letting the hardware implementation bleed into the frontend. We should do this in the backend instead and thereby simplify the interface.

The DummyStepperMotor class won't need changing.