MOBT-797: Coordinate retention in percentile generation

[bayliffe]

This PR enables the retention of scalar coordinates that are generated when collapsing coordinates to generate percentiles. It also ensures that if these retained coordinates are time related (time, forecast_period, forecast_reference_time), they are modified to ensure that the point is aligned to the upper bound, adhering with IMPROVER's metadata standards.

The purpose of this change is to enable the production of typical wind speed diagnostics with our existing tooling. The suite chain will become:

flowchart TD

A(wind speed in 3-hours) --> B(merge CLI: construct 24-hour cube with time coordinate) --> C(generate_percentiles CLI: collapse time and realization coordinates to generate percentiles)

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This change ensures that the time period covered by the diagnostic is available on the final diagnostic. Note that we will recycle the midnight validity time for adjacent days, e.g.

Day 1: 20250115T0000Z, 03, 06, 09, 12, 15, 18, 21, 20250116T0000Z
Day 2: 20250116T0000Z, 03, 06, 09, 12, 15, 18, 21, 20250117T0000Z

The wind speed diagnostics do no cover a period, so we use as many samples as we can for a given day. This means that, in the example shown here the 20250116T0000Z forecast is included in the forecast for day 1 and day 2. The generate_percentiles CLI will create bounds from the time points (as there are no bounds here), which results in bounds that span the whole 24-hours on the output cube.

Testing:

• Ran tests and they passed OK
• Added new tests for the new feature(s)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.40%. Comparing base (84a8944) to head (97ac22b).
Report is 64 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2087      +/-   ##
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+ Coverage   98.39%   98.40%   +0.01%     
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  Files         124      134      +10     
  Lines       12212    13254    +1042     
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+ Hits        12016    13043    +1027     
- Misses        196      211      +15     

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[Kat-90]

I tested this with a few coordinates to collapse and retain. It works as intended but I found that the bounds/points it puts on non-time related coordinates aren't always as useful. That's nothing to do with this PR though so happy to approve.

[brhooper]

Thanks @bayliffe, I've added a question about whether the CLI change requires an acceptance test to go along with it.