Add optional confidence

Related to issue #805 (closed)

The confidence field is present on endpoints CreateTranscriptionEntity and ListTranscriptionEntities. If not provided, it defaults to "null". Valid values are floats between 0 and 1.

This will be useful for NER models that outputs confidence score for entities. In the future we will also be able to filter entities based on this score.

arkindex/documents/api/entities.py

@@ -226,7 +226,7 @@ class EntityLinkCreate(CreateAPIView):
 ))
 class TranscriptionEntityCreate(CreateAPIView):
     """
-    Link an existing Entity to a given transcription with its position
+    Link an existing Entity to a given transcription with its position.
     """
     permission_classes = (IsVerified, )
     serializer_class = TranscriptionEntitySerializer