fixing kwargs behaviour when opening datatree stored in zarr

[aladinor]

• [x] Closes #327
• [x] Tests added
• [x] Passes pre-commit run --all-files
• [x] New functions/methods are listed in api.rst
• [x] Changes are summarized in docs/source/whats-new.rst

[TomNicholas]

Test looks great @aladinor - I think the error is just because dask is not installed in the test environment.

[aladinor]

Thanks @TomNicholas. Should we include it? do you think we can merge without passing the checks?

[TomNicholas]

Just try adding it to the environment.yml file and see if the automated checks pass.