feature 780 unsupported MET config overrides

[georgemccabe]

Pull Request Testing

• Describe testing already performed for these changes:
  
  Made sure all tests pass
  Tested out a few wrappers overriding variables and confirmed the values were successfully overridden

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  
  Get use case that needs to override climo field names to work

• Do these changes include sufficient documentation and testing updates? [Yes]
  I added the new config variables to the glossary and python wrappers pages in the User's Guide. More information on how to use this functionality seems like it would fit in the new sections that @DanielAdriaansen is adding as part of the other work relating to Update setting of environment variables for MET config files to add support for all to METPLUS_ vars #768

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

Pull Request Checklist

See the METplus Workflow for details.

• 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), Project(s), and Milestone
• After submitting the PR, select Linked Issues with the original issue number.
• 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.

[DanielAdriaansen]

@georgemccabe looks like this just involves some of the commits on #768 right? So I can continue working as I was planning on that branch?

[georgemccabe]

@DanielAdriaansen , I tried to cherry pick some of the commits from the branch you are working in so we can get this functionality tested and added to develop since other people need it for their cases. The commits here are slightly different than our branch because ours has additional functionality. Once this PR is merged into develop, there will probably be some minor conflicts to resolve when merging those changes into this branch. I can take care of that part.

Yes, you can continue working in the branch.

[j-opatz]

New functions here allowed for use case to run that needed MET config override options that are outside the standard currently used. String interpretation works as described, and utility of this kind is strongly needed.