allow to set exported properties in code

Actually, only property values set in the configuration can
be exported, as values equal to the default are not exported.
For this, the mechanism of overwriting properties by class attributes
has to be modified.

Change-Id: I4388d1fbb36393e863556fbbc8df800dd4800c87
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/22161
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop_psi/ppms.py

@@ -127,8 +127,6 @@ class Main(Communicator):
 class PpmsMixin(HasIodev, Module):
     properties = {
         'iodev': Attached(),
-        'general_stop': Property('respect general stop', datatype=BoolType(),
-                                 export=True, default=True)
     }
 
     pollerClass = Poller
@@ -439,6 +437,11 @@ class Temp(PpmsMixin, Drivable):
         14: [Status.ERROR, 'can not complete'],
         15: [Status.ERROR, 'general failure'],
     }
+    properties = {
+        'iodev': Attached(),
+        'general_stop': Property('respect general stop', datatype=BoolType(),
+                                 export=True, default=True)
+    }
 
     channel = 'temp'
     _stopped = False