Dataset add relative path to an image

[wuyuanyi135]

Hello,

Thanks for creating this handy package! I have a suggestion. Is it a good idea to have a base_dir argument to the class Dataset, so that Dataset.add(str) can handle the relative path given by os.path.join(base_dir, rel_path). When load from an external file, the image_file paths will be concatenated by the base_dir to find the correct path.

I think this would be useful if one wants to create a self-contained dataset with relative paths.

Thanks.