introduce update callbacks

includes a use case:
- a software calibration, to be applied to any Readable.
- calibration could be changed on the fly

+ refactored a little bit update events mechanism

Change-Id: Ifa340770caa9eb2185fe7e912c51bd9ddb411ece
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/23093
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/iohandler.py

@@ -286,7 +286,7 @@ class IOHandler(IOHandlerBase):
         except Exception as e:
             # set all parameters of this handler to error
             for pname in self.parameters:
-                module.setError(pname, e)
+                module.announceUpdate(pname, None, e)
             raise
         return Done