fix(relation): Add a "_bis" if relation already exists

This is the behavior for dataproperty and class, then let's use
the same strategy here

owl2yams/__init__.py

@@ -241,10 +241,7 @@
             print(f"Warning: object property {object_property_uri} could not be parsed")
             continue
         if object_property_uri_fragment in schema:
-            raise ValueError(
-                f"Error: relation {object_property_uri} already defined "
-                f"({relationtype_fragment_to_uri[object_property_uri_fragment]})"
-            )
+            object_property_uri_fragment += "_bis"
         schema.add_relation_type(RelationType(object_property_uri_fragment))
         relationtype_fragment_to_uri[object_property_uri_fragment] = object_property_uri
         all_types = list(entitytype_fragment_to_uri.keys())