#31 API POST Compensation

* adds initialize_objects to an abstractmethod of the super class to be implemented in subclasses
* differentiates error messages if intervention does not exist or is just not shared with the user

api/utils/serializer/serializer.py

@@ -133,4 +133,19 @@ class AbstractModelAPISerializer:
         return self.model.objects.get(
             id=id,
             users__in=[user]
-        )
+        )
+
+    @abstractmethod
+    def initialize_objects(self, json_model, user):
+        """ Initializes all needed objects from the json_model data
+
+        Does not persist data to the DB!
+
+        Args:
+            json_model (dict): The json data
+            user (User): The API user
+
+        Returns:
+            obj (Intervention)
+        """
+        raise NotImplementedError("Must be implemented in subclasses")