#138 WIP Validity

* adds geometry validity checks for SimpleGeomForm is_valid()
    * shows validity problems on the form if a feature is invalid
* optimizes merging of different features into one MultiPolygon
* further enhances tests
* adds as_feature_collection() method on Geometry model for converting geom MultiPolygon attribute into FeatureCollection json holding each polygon as an own feature -> makes each polygon selectable in new netgis map client

compensation/tests/compensation/test_workflow.py

@@ -128,6 +128,7 @@ class CompensationWorkflowTestCase(BaseWorkflowTestCase):
         new_identifier = self.create_dummy_string()
         new_comment = self.create_dummy_string()
         new_geometry = MultiPolygon(srid=4326)  # Create an empty geometry
+        geojson = self.create_geojson(new_geometry)
 
         check_on_elements = {
             self.compensation.title: new_title,
@@ -142,7 +143,7 @@ class CompensationWorkflowTestCase(BaseWorkflowTestCase):
             "title": new_title,
             "intervention": self.intervention.id,  # just keep the intervention as it is
             "comment": new_comment,
-            "geom": new_geometry.geojson,
+            "geom": geojson,
         }
         self.client_user.post(url, post_data)
         self.compensation.refresh_from_db()