Implement worker

Closes #2 (closed)

worker_generic_training_dataset/db.py

+# -*- coding: utf-8 -*-
+
+from uuid import UUID
+
+from arkindex_export import Classification
+from arkindex_export.models import (
+    Element,
+    Entity,
+    EntityType,
+    Transcription,
+    TranscriptionEntity,
+)
+
+
+def retrieve_element(element_id: UUID) -> Element:
+    return Element.get_by_id(element_id)
+
+
+def list_classifications(element_id: UUID):
+    return Classification.select().where(Classification.element == element_id)
+
+
+def list_transcriptions(element_id: UUID):
+    return Transcription.select().where(Transcription.element == element_id)
+
+
+def list_transcription_entities(transcription_id: UUID):
+    return (
+        TranscriptionEntity.select()
+        .where(TranscriptionEntity.transcription == transcription_id)
+        .join(Entity, on=TranscriptionEntity.entity)
+        .join(EntityType, on=Entity.type)
+    )