In `calculate_registration_image_based`, should we check that `zarr_url != reference_zarr_url`?

[jluethi]

It's mostly the job of the init function to not provide the reference_zarr_url as one of the parallelization items. If it does, nothing would break though (it's just a trivial registration that gets calculated).

We can add a check. In the past, we had something there to just skip the task if that was provided. Not sure whether this is still the most relevant