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>

test/test_iohandler.py

@@ -73,17 +73,17 @@ class DispatcherStub:
     def __init__(self, updates):
         self.updates = updates
 
-    def announce_update(self, moduleobj, pname, pobj):
-        self.updates[pname] = pobj.value
-
-    def announce_update_error(self, moduleobj, pname, pobj, err):
-        self.updates[('error', pname)] = str(err)
+    def announce_update(self, module, pname, pobj):
+        if pobj.readerror:
+            self.updates['error', pname] = str(pobj.readerror)
+        else:
+            self.updates[pname] = pobj.value
 
 
 class LoggerStub:
     def debug(self, *args):
         pass
-    info = exception = debug
+    info = warning = exception = debug
 
 
 class ServerStub: