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_statemachine.py

@@ -24,6 +24,7 @@
 from frappy.core import Drivable, Parameter
 from frappy.datatypes import StatusType, Enum
 from frappy.states import StateMachine, Stop, Retry, Finish, Start, HasStates, status_code
+from frappy.lib import generalConfig
 
 
 class LoggerStub:
@@ -189,9 +190,9 @@ class DispatcherStub:
     # initial value from the timestamp. However, in the test below
     # the second update happens after the updates dict is cleared
     # -> we have to inhibit the 'omit unchanged update' feature
-    omit_unchanged_within = 0
 
     def __init__(self, updates):
+        generalConfig.testinit(omit_unchanged_within=0)
         self.updates = updates
 
     def announce_update(self, modulename, pname, pobj):