Investigate and either fix or quell zarr.zip warnings

https://github.com/Deltares/imod-python/pull/1706 implements support for zarr and zarr.zip. The latter however gives quite some warnings about duplicate objects, which are indeed created:

There is some documentation on this: https://zarr.readthedocs.io/en/v2.15.0/api/storage.html#zarr.storage.ZipStore
We have to see if we can align chunks in some way to avoid this warning.

@Manangka also got the following warning which requires some investigation:

UnstableSpecificationWarning: The data type (FixedLengthUTF32(length=4, endianness='little')) does not have a Zarr V3 specification. That means that the representation of arrays saved with this data type may change without warning in a future version of Zarr Python. Arrays stored with this data type may be unreadable by other Zarr libraries. Use this data type at your own risk! Check https://github.com/zarr-developers/zarr-extensions/tree/main/data-types for the status of data type specifications for Zarr V3.
v3_unstable_dtype_warning(self)

Start by reviewing pull request #1706 and the linked Zarr ZipStore documentation. Reproduce the duplicate-object warnings and the UnstableSpecificationWarning, then determine whether chunk alignment or another documented approach addresses them. Done means the warnings are resolved or a clear decision explains why they are suppressed or retained.