wip API endpoints

arkindex/documents/fixtures/data.json

arkindex/documents/management/commands/build_fixtures.py

arkindex/process/migrations/0029_processdataset_sets.py

arkindex/process/models.py

arkindex/project/serializer_fields.py

arkindex/training/admin.py

arkindex/training/api.py

arkindex/training/migrations/0001_initial.py

arkindex/training/migrations/0007_datasetset_model.py

arkindex/training/models.py

arkindex/training/serializers.py

arkindex/training/tests/test_datasets_api.py