Fix warnings

[magnusuMET]

Change Summary

Enables warnings as errors, and fixes the warnings emitted during running of the tests.

Iris warning is still ignored, as the fix is not trivial. The relevant path is found in this part of the iris code, we could fix this by using a GeogCS coordinate system.

Related issue number

#1066

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.56%. Comparing base (c322808) to head (c8b2b28).
Report is 38 commits behind head on main-dev.

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1387      +/-   ##
============================================
+ Coverage     78.54%   78.56%   +0.01%     
============================================
  Files           137      137              
  Lines         20895    20905      +10     
============================================
+ Hits          16413    16424      +11     
+ Misses         4482     4481       -1     

Flag	Coverage Δ
unittests	78.56% <100.00%> (+0.01%)	⬆️

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[magnusuMET]

New contributors can't choose their reviewer. Maybe add a "ping @lewisblake if new to the project" should be added to the checklist?