parameters with a handler are polled by default

parameters with a common commandhandler need only one poller item

Change-Id: Ie2e20cf7bcebc67ca96d58455323f01f528b25ab
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21951
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_poller.py

@@ -87,6 +87,7 @@ class Parameter:
         self.readonly = readonly
         self.interval = interval
         self.timestamp = 0
+        self.handler = None
         self.reset()
 
     def reset(self):