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_demo/modules.py

@@ -120,6 +120,7 @@ class MagneticField(Drivable):
                      )
     heatswitch = Attached(Switch, description='name of heat switch device')
 
+    # pylint: disable=invalid-name
     Status = Enum(Drivable.Status, PERSIST=PERSIST, PREPARE=301, RAMPING=302, FINISH=303)
 
     status = Parameter(datatype=TupleOf(EnumType(Status), StringType()))
@@ -193,6 +194,7 @@ class MagneticField(Drivable):
         self.log.error(self, 'main thread exited unexpectedly!')
 
     def stop(self):
+        """stop at current value"""
         self.write_target(self.read_value())