Relative paths in `labels.json`

The path to the images contained in labels.json should be relative to labels.json.

When we need to move datasets, we should also remember to edit labels.json to rename the paths used. (This often happens when we move to JeanZay).

The paths should be relative to the labels.json file itself to avoid this step (this is what YOLO does).

We could also add a parameter to the command to give an absolute path in case the labels.json file has been moved.

This needs a patch in dan.dataset.download.images