Improve utility of warnings in Iris tests

[trexfeathers]

Story

• As a
  Iris developer
• I want
  Warnings that are a problem for Iris to result in CI failure (e.g. a DeprecationWarning)
• So that
  I can address problems introduced by new commits proactively and immediately (rather than reactively when issues are discovered downstream)

Notes

#4384

Iris Gallery includes (or has included!) code to convert warnings to failures/errors.

Ideas

• Suppress warnings we know are not a problem, so only those that matter are left. Then set a test run to exit non-0 if any warnings are raised. Would likely need to address quite a few warnings before this could start working!
• A custom routine that causes a failure if it detects warnings of known categories (might only be DeprecationWarning?).

From discussion

• Cost-benefit is difficult. Focussing on DeprecationWarning would be fairly easy, but is it offering much. Covering all warnings may need a lot of leg work to adapt the tests.
• Should start by seeing how many tests fail if warnings are 'promoted', and see how this informs further discussions.

Tasks

Beta Give feedback

1. Create proof of concept: Iris tests fail if any warnings raised #5465
   +0
   
2. pkg_resources is deprecated, reccomends porting to importlib.resources #5491
   Type: Infrastructure +1
   
3. DeprecationWarning in pp file saving #5492
   Type: Bug +1
4. Updated all np.product calls to np.prod #5493
   +0
5. Resolve runtime warning: invalid value encountered in sqrt. #5535
   Type: Infrastructure +1
6. Resolve runtime warning: divide by zero encountered in divide #5534
   Type: Infrastructure +1
7. Resolve runtime warning: invalid value encountered in cast #5533
   Type: Infrastructure +1
8. Resolve numpy deprecation: using large arrays as indexes for slicing #5530
   Type: Infrastructure +1
9. Resolve numpy deprecation: array conversion into scalar. #5531
   Type: Infrastructure +1
10. Resolve numpy deprecation: using matrix subclass instead of recommended ndarray #5532
    Type: Infrastructure +1
11. Try setting PyTest to 'filter out' any IrisUserWarnings #5889
    Type: Infrastructure Type: Testing +2

[ESadek-MO]

Note: have converted write up from #5465 into issues and added as subtasks on this issue.