fix subset level by values with method=nearest

[cehbrecht]

Pull Request Checklist:

• [ ] This PR addresses an already opened issue (for bug fixes / features)

  • This PR fixes issue #xyz

• [ ] Tests for the changes have been added (for bug fixes / features)

• [ ] Documentation has been added / updated (for bug fixes / features)

• [ ] HISTORY.rst has been updated (with summary of main changes)

• [x] I have added my relevant user information to AUTHORS.md

• What kind of change does this PR introduce?: Bug fix

• Does this PR introduce a breaking change?: No

• Other information:

subset_level_by_values didn't work for CMIP6 dataset. Needed to add method="nearest" to ds.sel to make it work.

The example notebook (Process error: Requested levels include some not found in the dataset: 1000.0): https://nbviewer.org/github/roocs/rooki/blob/master/notebooks/demo/rook-errors-2022-12-08.ipynb