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/modules.py

@@ -36,6 +36,7 @@ from .modulebase import Module
 
 class Readable(Module):
     """basic readable module"""
+    # pylint: disable=invalid-name
     Status = Enum('Status',
                   IDLE=StatusType.IDLE,
                   WARN=StatusType.WARN,
@@ -92,7 +93,7 @@ class Drivable(Writable):
 
     @Command(None, result=None)
     def stop(self):
-        """cease driving, go to IDLE state"""
+        """not implemented - this is a no-op"""
 
 
 class Communicator(HasComlog, Module):