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/modules.py

@@ -160,7 +160,7 @@ class Module(HasProperties, metaclass=ModuleMeta):
                 if paramobj:
                     if propname == 'datatype':
                         paramobj.setProperty('datatype', get_datatype(cfgdict.pop(k)))
-                    elif propname in paramobj.__class__.properties:
+                    elif propname in paramobj.getProperties():
                         paramobj.setProperty(propname, cfgdict.pop(k))
                     else:
                         raise ConfigError('Module %s: Parameter %r has no property %r!' %
@@ -208,8 +208,9 @@ class Module(HasProperties, metaclass=ModuleMeta):
 
         # Modify units AFTER applying the cfgdict
         for k, v in self.parameters.items():
-            if '$' in v.unit:
-                v.unit = v.unit.replace('$', self.parameters['value'].unit)
+            dt = v.datatype
+            if '$' in dt.unit:
+                dt.setProperty('unit', dt.unit.replace('$', self.parameters['value'].datatype.unit))
 
         # 6) check complete configuration of * properties
         self.checkProperties()