Replace loading `.zattrs` file with accessing zarr attrs

[tcompa]

We currently do it at least in the following cases:

$ git grep -n "with open(" | grep zattr

lib_zattrs_utils.py:36:    with open(zattrs_path, "r") as jsonfile:
tasks/cellpose_segmentation.py:397:    with open(zattrs_file, "r") as jsonfile:
tasks/cellpose_segmentation.py:430:        with open(f"{zarrurl}/labels/.zattrs", "r") as f_zattrs:
tasks/napari_workflows_wrapper.py:200:    with open(zattrs_file, "r") as jsonfile:
tasks/napari_workflows_wrapper.py:431:            with open(f"{zarrurl}/labels/.zattrs", "r") as f_zattrs:

while it'd be more general to directly access the attrs of a zarr group, rather than depending on a specific on-disk storage.

[jluethi]

Very good idea to shift this to the zarr group! Feel free to tag that in the registration PR as well, there may be one or another of those there as well :)

[tcompa]

After #528, this issue will appear only in a few places.

• Update get_table_path_dict to take a zarr group rather than a path
• Update apply_registration_to_image.py to access labels via the appropriate Ngff object
• Update napari_workflows_wrapper.py, re: multiscales

EDIT: as of c44ef38, all three points are fixed.

[tcompa]

A small fix to the cellpose was still missing. Done with 3d62b29.