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>

doc/source/programming.rst

@@ -31,9 +31,6 @@ def read\_\ *<parameter>*\ (self):
     Called on a ``read`` SECoP message and whenever the internal poll mechanism
     of Frappy tries to get a new value. The return value should be the
     retrieved value.
-    In special cases :data:`Done <frappy.modules.Done>` might be returned instead,
-    when the internal code has already updated the parameter, or
-    when the value has not changed and no updates should be emitted.
     This method might also be called internally, in case a fresh value of
     the parameter is needed.
 
@@ -55,9 +52,6 @@ def write\_\ *<parameter>*\ (self, value):
     value would be the same, as if it would be done by the ``read_<parameter>``
     method. Often the easiest implementation is just returning the result of
     a call to the ``read_<parameter>`` method.
-    Also, :ref:`Done <done unique>` might be returned in special
-    cases, e.g. when the code was written in a way, when self.<parameter> is
-    assigned already before returning from the method.
 
 .. admonition:: behind the scenes