feature 986 TCPairs support LOOP_BY = VALID when LOOP_ORDER = processes

[georgemccabe]

Modified logic to allow loop by valid time for TCPairs when loop order = PROCESSES.

Pull Request Testing

• Describe testing already performed for these changes:
  

Modified unit test logic to test when loop_order = PROCESSES and LOOP_BY = VALID

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

Review code changes and ensure no differences in automated tests.

• 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]
  

• Please complete this pull request review by 7/7/2021.
  

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[JohnHalleyGotway]

Per our discussion on 7/7/21, recommend that you:

• retract this current PR
• enhance the feature branch to use the lead list to process tracks from all initialization times, init_time = valid_time - lead_list[i]
• checkout this feature branch on jet and ask @mollybsmith-noaa to test out these changes there

@mollybsmith-noaa please feel free to schedule a meeting with George and I next week to talk through these details more closely as you see fit.

Once @mollybsmith-noaa confirms that these changes meet her need, resubmit a PR.

[georgemccabe]

Closing the PR to make additional changes.