changed atomic access for updating the acceleration profile

the stepper interrupt is only halted when necessary and for the shortest
time possible (8% cycle time)

stepper.h

@@ -45,6 +45,7 @@ void st_cycle_reinitialize();
 // Initiates a feed hold of the running program
 void st_feed_hold();
 
+// Accessor function to query the acceleration state of the stepper
 uint8_t st_is_decelerating();
 
 #endif