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/params.py

@@ -31,8 +31,6 @@ from secop.datatypes import BoolType, CommandType, DataType, \
 from secop.errors import BadValueError, ProgrammingError
 from secop.properties import HasProperties, Property
 
-UNSET = object()  # an argument not given, not even None
-
 
 class Accessible(HasProperties):
     """base class for Parameter and Command