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_psi/channelswitcher.py

@@ -39,7 +39,7 @@ switcher=sw
 import time
 
 from frappy.datatypes import IntRange, BoolType, FloatRange
-from frappy.core import Attached, Property, Drivable, Parameter, Readable, Done
+from frappy.core import Attached, Property, Drivable, Parameter, Readable
 
 
 class ChannelSwitcher(Drivable):
@@ -55,6 +55,7 @@ class ChannelSwitcher(Drivable):
     """
     value = Parameter('the current channel number', IntRange(), needscfg=False)
     target = Parameter('channel to select', IntRange(), needscfg=False)
+    status = Parameter(update_unchanged='never')
     autoscan = Parameter('whether to scan automatically',
                          BoolType(), readonly=False, default=True)
     pollinterval = Parameter(default=1, export=False)
@@ -108,7 +109,7 @@ class ChannelSwitcher(Drivable):
         if self.status[0] == 'BUSY':
             chan = self._channels[self.target]
             if chan.is_switching(now, self._start_switch, self.switch_delay):
-                return Done
+                return self.status
             self.setFastPoll(False)
             self.status = 'IDLE', 'measure'
             self.value = self.target
@@ -116,7 +117,7 @@ class ChannelSwitcher(Drivable):
             chan.read_value()
             chan.read_status()
             if self.measure_delay > self._time_tol:
-                return Done
+                return self.status
         else:
             chan = self._channels[self.value]
             self.read_value()  # this might modify autoscan or deadline!
@@ -129,7 +130,7 @@ class ChannelSwitcher(Drivable):
                     chan.read_status()
                     self._last_measure = next_measure
                 if not self.autoscan or now + self._time_tol < self._start_measure + self.measure_delay:
-                    return Done
+                    return self.status
         next_channel = self.next_channel(self.value)
         if next_channel == self.value:
             return 'IDLE', 'single channel'