Make ASEM download more efficient

[constantinpape]

The download for ASEM currently downloads the whole volumes from S3, but the labels are restricted to a smaller subset in them. We should update the dataset so that we only download the ROIs with labels. (Which can be done using zarr with s3fs).

See https://github.com/constantinpape/torch-em/blob/main/torch_em/data/datasets/asem.py.