fix command doc string handling and change default stop doc string

- fix inheritance of command description
- when no stop method is given, then the description should indicate
  that stop is a no-op -> add missing doc strings to stop methods
- add test to make sure stop command doc strings are given
  when implemented

Change-Id: If891359350e8dcdec39a706841d61d4f8ec8926f
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/33266
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

frappy_psi/ppms.py

@@ -483,6 +483,10 @@ class Temp(PpmsDrivable):
         self._expected_target_time = time.time() + abs(target - self.value) * 60.0 / max(0.1, ramp)
 
     def stop(self):
+        """set setpoint to current value
+
+        but restrict to values between last target and current target
+        """
         if not self.isDriving():
             return
         if self.status[0] != StatusType.STABILIZING:
@@ -612,6 +616,7 @@ class Field(PpmsDrivable):
         # do not execute FIELD command, as this would trigger a ramp up of leads current
 
     def stop(self):
+        """stop at current driven Field"""
         if not self.isDriving():
             return
         newtarget = clamp(self._last_target, self.value, self.target)
@@ -714,6 +719,7 @@ class Position(PpmsDrivable):
         return value  # do not execute MOVE command, as this would trigger an unnecessary move
 
     def stop(self):
+        """stop motor"""
         if not self.isDriving():
             return
         newtarget = clamp(self._last_target, self.value, self.target)