Decode CF metadata for pre-opened datasets

[Zaczero]

Inspired by #702, _open_nc_file would fail to decode types when xr.Dataset was passed.

• Tests added
• Tests passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.68%. Comparing base (4da28b0) to head (21688a8).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #706      +/-   ##
==========================================
+ Coverage   93.67%   93.68%   +0.01%     
==========================================
  Files          89       89              
  Lines       13621    13656      +35     
==========================================
+ Hits        12759    12794      +35     
  Misses        862      862              

Flag	Coverage Δ
unittests	93.68% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[djhoese]

I think this makes sense to me. Thanks for the improvement. @pnuu @mraspaud or anyone else have opinions? It would make sense that if you want to load an area from a CF dataset that you need to actually decode CF information first.

[djhoese]

Thanks @Zaczero!