Offline processing Classifier

Depends #17 (closed)

We now need an offline YOLO classifier. Write a new module in worker_yolo/offline to implement this worker. You should try to inherit as much code as possible from the actual classifier. You will need to

• isolate the image loading to avoid doing it in your worker,
• override the create_classification method.

This worker will create a JSON with two keys, ml_class and confidence with these values. This file will be stored under the name self.task_data_dir / self.element_id.

Refs https://redmine.teklia.com/issues/5020