Fix sdba_encode_cf bug

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

In xarray#7494 the _contains_cftime_datetimes function was restricted to Variable objects. We were passing a DataArray to xr.conventions.encode_cf_variable , which was passing it (assuming a variable) to that function. Since the PR, this raises an error.

In a way, this is my error because the encode_cf_variable clearly documents that its input should be a Variable, although there were not annotations at the time I implemented that line.

Moreover, we were not testing this code... It depended on the sdba_encode_cf option being activated, but no test was activating it.

This PR passes Variable instances were it should and enables the option in one of the test where it makes sense.

Does this PR introduce a breaking change?

No.

Other information:

Fixes xscen#175.

[Zeitsperre]

Merge when ready!