add deterministic verification functions from vtb to earthkit-meteo

[Oisin-M]

Description

Work towards #94.

TODOs:

• docstrings (with formulae)
• tests (failing tests are unrelated to this PR)
• credit scores somewhere
• would be nice to have: document nan handling

Some open questions:

• should all functions accept agg_dims etc.?
• function signature decisions - is over a good name?
• is this the correct place to put all the verification functions?

Notes for future:

• xarray metadata - should we rename the variable etc.? (future PR)

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.40%. Comparing base (36e7b50) to head (366a4b9).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #95      +/-   ##
===========================================
+ Coverage    96.97%   99.40%   +2.42%     
===========================================
  Files           21       22       +1     
  Lines         1556     1855     +299     
  Branches        33       35       +2     
===========================================
+ Hits          1509     1844     +335     
+ Misses          42        5      -37     
- Partials         5        6       +1     

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[oiffrig]

Looking good! I'm happy to see this grow :)