New DatasetSet model

closes #1712 (closed)

TODO:

• create process dataset issue
• create new API endpoints issue

arkindex/documents/export/dataset.sql

@@ -2,6 +2,8 @@ SELECT
     dataset.id,
     dataset.name,
     dataset.state,
-    ARRAY_TO_STRING(dataset.sets, ',', '')
+    string_agg(datasetset.name, ',')
 FROM training_dataset dataset
+INNER JOIN training_datasetset datasetset ON datasetset.dataset_id = dataset.id
 WHERE dataset.corpus_id = '{corpus_id}'::uuid
+GROUP BY dataset.id