fix batch unstacking for different NWP sources

[dfulu]

Pull Request

Description

Fix unstack_np_batch_into_examples() for different NWP sources. Also extend test so it would have caught this.

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked my code and corrected any misspellings

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (702542b) 77.51% compared to head (8e5dfa9) 77.51%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #259 +/- ## ======================================= Coverage 77.51% 77.51% ======================================= Files 118 118 Lines 5364 5364 ======================================= Hits 4158 4158 Misses 1206 1206 ```

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