make return value 'Done' unneccessary

'Done' was introduced in order to suppress unneccessary
duplicate updates. However, since super calls on access methods are
allowed, it is not nice when such a method returns Done, as this
is not automagically replaced by the current parameter value.
As a consequence:

- using Done is discouraged, but not (yet) removed in all code
- the 'omit_unchanged_within' property is moved from Module to an
  internal Parameter property 'update_unchanged'
- its default is moved from a SEC node property to generalConfig
- the 'update_unchanged' parameter property may be set to
  'never' for parameters where duplicate updates make no sense
- this property might be set to 'always', for measurements, where
  even unchanged values taken from HW should be transmitted

Change-Id: I2847c983ca09c2c4098e402edd08d0c96c3913f4
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/30672
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_poller.py

@@ -31,6 +31,7 @@ import pytest
 
 from frappy.core import Module, Parameter, FloatRange, Readable, ReadHandler, nopoll
 from frappy.lib.multievent import MultiEvent
+from frappy.lib import generalConfig
 
 
 class Time:
@@ -66,13 +67,14 @@ class DispatcherStub:
 
 class ServerStub:
     def __init__(self):
+        generalConfig.testinit()
         self.dispatcher = DispatcherStub()
 
 
 class Base(Module):
     def __init__(self):
         srv = ServerStub()
-        super().__init__('mod', logging.getLogger('dummy'), dict(description=''), srv)
+        super().__init__('mod', logging.getLogger('dummy'), {'description': ''}, srv)
         self.dispatcher = srv.dispatcher
 
     def run(self, maxcycles):