feat: Import yolo-seg labels. Includes conversion from bboxes to polygons if user is wishing to transition dataset.

[carbaro]

Hi

This PR seamlessly integrates with the current yolo.py imports module. Based on the yolo_type option, it modifies the content of the task json correspondingly.

Further, it allows:

• datasets split in subdirectories (e.g. train/val with images/labels each)
• prediction confidence values with the option --out_type
• polygonises bboxes automatically
• implements test where test data is polygonised and converted into JSON