allow to set exported properties in code

Actually, only property values set in the configuration can
be exported, as values equal to the default are not exported.
For this, the mechanism of overwriting properties by class attributes
has to be modified.

Change-Id: I4388d1fbb36393e863556fbbc8df800dd4800c87
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/22161
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_properties.py

@@ -133,15 +133,6 @@ def test_Property_override():
                 pass
     assert 'collides with method' in str(e.value)
 
-    with pytest.raises(ProgrammingError) as e:
-        class cy(c): # pylint: disable=unused-variable
-            properties = {
-                'b' : Property('', FloatRange(), 3.14),
-            }
-            b = 1.5
-
-    assert 'name collision with property' in str(e.value)
-
     with pytest.raises(ProgrammingError) as e:
         class cz(c): # pylint: disable=unused-variable
             a = 's'