Create is_finetuning property in TrainingMixin

This new property returns a boolean, True if self.model_version_id is set, False otherwise.

Defined in https://gitlab.teklia.com/workers/base-worker/-/blob/5857def9bff2e7340e7caa521b24af6eaded630f/arkindex_worker/worker/training.py#L74

changed milestone to %0.3.7

added P1 Quick Win labels

assigned to @mblanco

mentioned in merge request !501 (merged)

closed with merge request !501 (merged)