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>

frappy/io.py

@@ -33,7 +33,7 @@ from frappy.datatypes import ArrayOf, BLOBType, BoolType, FloatRange, IntRange,
     StringType, TupleOf, ValueType
 from frappy.errors import CommunicationFailedError, ConfigError, ProgrammingError
 from frappy.modules import Attached, Command, \
-    Communicator, Done, Module, Parameter, Property
+    Communicator, Module, Parameter, Property
 from frappy.lib import generalConfig
 
 generalConfig.set_default('legacy_hasiodev', False)
@@ -110,7 +110,8 @@ class IOBase(Communicator):
                    """, datatype=StringType())
     timeout = Parameter('timeout', datatype=FloatRange(0), default=2)
     wait_before = Parameter('wait time before sending', datatype=FloatRange(), default=0)
-    is_connected = Parameter('connection state', datatype=BoolType(), readonly=False, default=False)
+    is_connected = Parameter('connection state', datatype=BoolType(), readonly=False, default=False,
+                             update_unchanged='never')
     pollinterval = Parameter('reconnect interval', datatype=FloatRange(0), readonly=False, default=10)
     #: a dict of default settings for a device, e.g. for a LakeShore 336:
     #:
@@ -151,20 +152,20 @@ class IOBase(Communicator):
         self.is_connected is changed only by self.connectStart or self.closeConnection
         """
         if self.is_connected:
-            return Done  # no need for intermediate updates
+            return True
         try:
             self.connectStart()
             if self._last_error:
                 self.log.info('connected')
                 self._last_error = 'connected'
                 self.callCallbacks()
-                return Done
+                return self.is_connected
         except Exception as e:
             if str(e) != self._last_error:
                 self._last_error = str(e)
                 self.log.error(self._last_error)
             raise SilentError(repr(e)) from e
-        return Done
+        return self.is_connected
 
     def write_is_connected(self, value):
         """value = True: connect if not yet done