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_params.py

@@ -109,3 +109,21 @@ def test_Export():
     class Mod(HasAccessibles):
         param = Parameter('description1', datatype=BoolType, default=False)
     assert Mod.param.export == '_param'
+
+
+@pytest.mark.parametrize('arg, value', [
+    ('always', 0),
+    (0, 0),
+    ('never', 999999999),
+    (999999999, 999999999),
+    (1, 1),
+])
+def test_update_unchanged_ok(arg, value):
+    par = Parameter('', datatype=FloatRange(), default=0, update_unchanged=arg)
+    assert par.update_unchanged == value
+
+
+@pytest.mark.parametrize('arg', ['alws', '', -2, -0.1, None])
+def test_update_unchanged_fail(arg):
+    with pytest.raises(ProgrammingError):
+        Parameter('', datatype=FloatRange(), default=0, update_unchanged=arg)