Feature #2827 SeriesAnalysis gradient

[georgemccabe]

Also added support for setting gradient dictionary in GridStat wrapper because it was not supported.

Pull Request Testing

• Describe testing already performed for these changes:
  

Added unit tests to confirm expected behavior

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

• Review code/doc changes
• Optionally run a use case with these changes setting gradient and confirm that it produces the expected results. Note these changes require the changes from PR Feature #3024 and #3030 Series-Analysis GRAD MET#3036 to be available

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes]

• Do these changes include sufficient testing updates? [Yes]

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Do these changes introduce new SonarQube findings? [No]
  
  If yes, please describe:

• Please complete this pull request review by Friday 1/17/25.
  

Pull Request Checklist

See the METplus Workflow for details.

• Add any new Python packages to the METplus Components Python Requirements table.
• For any new datasets, an entry to the METplus Verification Datasets Guide.
• Review the source issue metadata (required labels, projects, and milestone).
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• Ensure the PR title matches the feature or bugfix branch name.
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  Select: Reviewer(s) and Development issue
  Select: Milestone as the version that will include these changes
  Select: Coordinated METplus-X.Y Support project for bugfix releases or METplus-Wrappers-X.Y.Z Development project for official releases
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[j-opatz]

As noted in the PR, this issue piggybacks on the 3036 PR in MET. Assuming the underlying coding is correct, this PR's coding is sufficient to bring the capability to METplus wrappers and is approved. There were no compiling differences from GHA, as expected.