Fix path/to/image when exporting Plate images

[will-moore]

Fixes a bug introduced by #144 where the well/field/image path wasn't being passed to da.to_zarr() when downsampling images during Plate export.

To test:

omero zarr export Plate:123

This bug didn't affect the export of single Images (which was tested on that PR).
Without this fix, the first downsampled image 1 gets written to plate.zarr/1/.zarray and then this is not found when we try to read it to downsample again (since we're still looking in the "right" place at plate.zarr/A/1/0/1/.zarray):

Traceback (most recent call last):
  File "/Users/wmoore/opt/anaconda3/envs/omeroweb2/lib/python3.9/site-packages/zarr/core.py", line 243, in _load_metadata_nosync
    meta_bytes = self._store[mkey]
  File "/Users/wmoore/opt/anaconda3/envs/omeroweb2/lib/python3.9/site-packages/zarr/storage.py", line 1087, in __getitem__
    raise KeyError(key)
KeyError: '.zarray'

[joshmoore]

Since gh-147 is based on this PR, taking @dominikl's #147 (comment) as a sign-off.