Feature #2460 allow missing input

[georgemccabe]

Pull Request Testing

• [X] Describe testing already performed for these changes:
  

Added unit tests for new capability to allow missing input files and still run without error Ensured all existing tests and use cases pass

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

@JohnHalleyGotway and I met to discuss the details of this PR because there are a lot of changes. All tests should pass with no differences.

Review beta3 release notes: https://metplus--2493.org.readthedocs.build/en/2493/Users_Guide/release-notes.html#metplus-version-6-0-0-beta-3-release-notes-2024-02-07

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

Additional changes should be made after the beta3 release, including adding documentation for the new config variables. I created issue #2492 to complete remaining tasks.

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

Added and enhanced unit tests

• [X] Will this PR result in changes to the test suite? [No]
  

• [X] Please complete this pull request review by 2/7/2024.
  

Pull Request Checklist

See the METplus Workflow for details.

• [X] Add any new Python packages to the METplus Components Python Requirements table.
• [X] Review the source issue metadata (required labels, projects, and milestone).
• [X] Complete the PR definition above.
• [X] Ensure the PR title matches the feature or bugfix branch name.
• [X] 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
• [x] After submitting the PR, select the :gear: icon in the Development section of the right hand sidebar. Search for the issue that this PR will close and select it, if it is not already selected.
• [ ] 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.

[georgemccabe]

@JohnHalleyGotway, I will fix the typos in the release notes in the PR for creating the release.