Revise differential slewing

A contribution in the forum mentions a problem with differential slewing. The analysis of the program flow in align module shows a heavy flaw: Differential slewing ends prematurely, there is no iteration and the position threshold is never reached! (Just let the simulator do a differential slew and see how the position error remains red.) This MR rectifies the program flow and corrects at the same time the above mentioned problem with the rotator.