Adding support for Zarr stitching/fusion

[jwong-nd]

Hello, this PR adds zarr_processor.py, an entrypoint for processing Zarr datasets. zarr_processor.py imports:

• zarr_utils.py for general data loading
• coarse_registration.py for initial coarse tile registration
• fine_registration.py for elastic registration
• processor.warp.py for fusion

In addition, I tweaked fine_registration.py and processor.warp.py (deleting unused fields, renaming variables) for readability.

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[mjanusz]

Could you please also:

1. squash all the changes into a single commit,
2. on the new files that you're adding, run pyink with the settings from https://github.com/google-research/sofima/blob/main/pyproject.toml ?