Support giving a CachedElement in helper functions using the local cache

Depends on !88 (merged)

Closes #55 (closed)

arkindex_worker/worker/classification.py

@@ -4,6 +4,7 @@ import os
 from apistar.exceptions import ErrorResponse
 
 from arkindex_worker import logger
+from arkindex_worker.cache import CachedElement
 from arkindex_worker.models import Element
 
 
@@ -65,8 +66,8 @@ class ClassificationMixin(object):
         Create a classification on the given element through API
         """
         assert element and isinstance(
-            element, Element
-        ), "element shouldn't be null and should be of type Element"
+            element, (Element, CachedElement)
+        ), "element shouldn't be null and should be an Element or CachedElement"
         assert ml_class and isinstance(
             ml_class, str
         ), "ml_class shouldn't be null and should be of type str"