Add data_type to open_data method in DataStore class

[konstntokas]

Closes #1030

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[forman]

Very good work, thanks! See my change requests and please adjust accordingly.

[forman]

Adding keyword data_type is a change that breaks compatibility with any data store developed against older versions of xcube.

We should instead document the parameter here - as you did - and update data store implementations subsequently. Only after we have migrated all of our own data stores and after next xcube major release, we should define the kwarg here as you did.

Therefore, the xcube-stac store should pop data_type out of the open_params for time being.

[forman]

Suggested change

	* Added `data_type` to `open_data` method in the `DataStore` class, which determines
	the return value of the data set. Note that `opener_id` includes the `data_type`
	at its first position and will override the `date_type` argument. (#1030)
	* The `open_data` method of xcube's default `xcube.core.store.DataStore` implementations
	now support a keyword argument `data_type`, which determines
	data type of the return value. Note that `opener_id` includes the `data_type`
	at its first position and will override the `date_type` argument.
	To preserve backward compatibility, the the keyword argument `data_type`
	has not yet been added to the `open_data()` method arguments. (#1030)

[forman]

Suggested change

	If *data_type* is provided, the return value will be in the specified
	Implementations are advised to support an additional optional keyword
	argument `data_type: DataTypeLike = None`.
	If *data_type* is provided, the return value should be in the specified

[forman]

FYI @TonioF (xcube-cci), @pont-us (xcube-cds), @TejasMorbagal (xcube-cmems)

[konstntokas]

As suggested, I included in keyword argument data_type in **open_params now. Further I added testing to the open_data method in the ReferenceDataStore class. Please check if I understood correctly, that the open_data method in the ReferenceDataStore class only returns xr.Dataset. However most of these tests are skipped because kerchunk is not installed (see e.g. https://github.com/xcube-dev/xcube/blob/main/test/core/store/ref/test_store.py#L178). Should we maybe add kerchunk to the testing section in the environment.yml ?

[forman]

Please check if I understood correctly, that the open_data method in the ReferenceDataStore class only returns xr.Dataset.

True.

However most of these tests are skipped because kerchunk is not installed (see e.g. https://github.com/xcube-dev/xcube/blob/main/test/core/store/ref/test_store.py#L178). Should we maybe add kerchunk to the testing section in the environment.yml ?

Let's do that!