Draft: Refactor and implement API version of the worker

Closes #17

worker_generic_training_dataset/__init__.py

@@ -159,10 +159,13 @@ class Extractor(DatasetWorker):
         polygon = element.polygon
 
         if isinstance(element, Element):
+            # SQL result
             image = element.image
             wk_version = get_object_id(element.worker_version)
             wk_run = get_object_id(element.worker_run)
         else:
+            # API result
+            polygon = json.dumps(polygon)
             image = element.zone.image
             wk_version = (
                 element.worker_version