UniqueObject

create a class to be used for unique objects.
better for debugging and documentation than just using object()

+ remove unused unique objects

Change-Id: I32f65960ea2fbee4fccbeb49a4e11176b7185aa0
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/27455
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/modules.py

@@ -31,14 +31,17 @@ from secop.datatypes import ArrayOf, BoolType, EnumType, FloatRange, \
     IntRange, StatusType, StringType, TextType, TupleOf
 from secop.errors import BadValueError, ConfigError, \
     ProgrammingError, SECoPError, SilentError, secop_error
-from secop.lib import formatException, mkthread
+from secop.lib import formatException, mkthread, UniqueObject
 from secop.lib.enum import Enum
 from secop.params import Accessible, Command, Parameter
 from secop.poller import BasicPoller, Poller
 from secop.properties import HasProperties, Property
 from secop.logging import RemoteLogHandler, HasComlog
 
-Done = object()  #: a special return value for a read/write function indicating that the setter is triggered already
+Done = UniqueObject('already set')
+"""a special return value for a read/write function
+
+indicating that the setter is triggered already"""
 
 
 class HasAccessibles(HasProperties):