use Properties from secop.properties for datatypes

this change is triggered by the fact, that assigining a unit
in the config file did no longer work.

this change has several implications:
1) secop.properties must not import secop.datatypes:
   - as ValueType can not be imported, the default behaviour with
     'mandatory' and 'default' arguments was slightly changed
   - instead of checking for DataType when exporting, a try/except
     was used
2) the datatype of datatype properties is sometimes not yet defined.
   a stub is used in this cases instead, which is later replaced by
   the proper datatype. The number of stubs may be reduced, but this
   should be done in a later change, as the diff will be much less
   readable.
3) in config files, datatype properties can be changed like parameter
   properties. HasProperties.setProperties/checkProperties/getProperties
   are overridden for this.

the config editor seems still to work, an issue (probably py3) had
to be fixed there

Change-Id: I1efddf51f2c760510e913dbcaa099e8a89c9cab5
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21399
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/metaclass.py

@@ -199,6 +199,6 @@ class ModuleMeta(PropertyMeta):
         # collect info about parameters and their properties
         for param, pobj in cls.accessibles.items():
             res[param] = {}
-            for pn, pv in pobj.__class__.properties.items():
+            for pn, pv in pobj.getProperties().items():
                 res[param][pn] = pv
         return res