added variable selection to make it compatible with sarah3

[matzech]

Change proposed in this Pull Request

I noticed that the code is not compatible with the novel SARAH-3 dataset as the input data has changed slightly.When reading with the existing code, it results into multiple variables which raises errors in the downstream conversion. Therefore, this fork propose to directly select the respective SIS and SID values making it compatible with the old and the new SARAH format. It uses double brackets to keep the same data type (xr.DataSet) as so far.

Description

Only changed the lines that read the SARAH netcdf files by directly selecting the data variables.

Motivation and Context

I had an issue opening SARAH -3 data with the existing code.

How Has This Been Tested?

I tested it with the old SARAH -2 data and the novel SARAH-3. Both works as expected. I was not sure if I should add unit tests because there are, as far as I have seen, no unit tests for the different datasets so far.

Type of change

• [x ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x ] I tested my contribution locally and it seems to work fine.
• [ ] I locally ran pytest inside the repository and no unexpected problems came up.
• [ ] I have adjusted the docstrings in the code appropriately.
• [ ] I have documented the effects of my code changes in the documentation doc/.
• [ ] I have added newly introduced dependencies to environment.yaml file.
• [ ] I have added a note to release notes doc/release_notes.rst.
• [ ] I have used pre-commit run --all to lint/format/check my contribution

[fneum]

LGTM, maybe add a short release note announcing compatibility with SARAH-3. Unit test not necessary since we have none for SARAH-2 so far. Does the selection with double brackets preserve lazy loading?