Feature 685 group pytests

[georgemccabe]

Change Summary:

• Created markers to group pytests
• Enhanced unit test logic to run each group of pytests instead of all at once (see notes in Internal: Improve use case testing #685)
• Marked all unit tests with a custom marker to group tests
• Cleaned up test logic including removing unused imports and removing unnecessary parenthesis in assert statements

Note some changes to the GHA logic are not necessary for the final solution that was implemented, but I left those changes in place because they don't hurt and are necessary if we ever decide to split the pytest job into multiple jobs in the future.

Pull Request Testing

• Describe testing already performed for these changes:
  

Ensured all tests pass and time to run the pytest job was greatly reduced (from ~30m to ~10m)

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

• Review changes to automation logic in case in case similar logic could benefit other METplus components
• Ensure all automated test checks pass

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

Added info to Contributor's Guide about new functionality

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

• Will this PR result in changes to the test suite? [No]
  
  The test suite should take less time to run, but everything should still pass as usual.

• Please complete this pull request review by 7/13/2022.
  

Pull Request Checklist

See the METplus Workflow for details.

• Add any new Python packages to the METplus Components Python Requirements table.
• Review the source issue metadata (required labels, projects, and milestone).
• Complete the PR definition above.
• Ensure the PR title matches the feature or bugfix branch name.
• Define the PR metadata, as permissions allow.
  Select: Reviewer(s)
  Select: Organization level software support Project or Repository level development cycle Project
  Select: Milestone as the version that will include these changes
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• After the PR is approved, merge your changes. If permissions do not allow this, request that the reviewer do the merge.
• Close the linked issue and delete your feature or bugfix branch from GitHub.

[hankenstein2]

Everything ran well on seneca. I think this will be useful for METplotpy and METcalcpy but might take us a bit to incorporate.