Feature 461 make compare images configurable

[John-Sharples]

Makes the use of CompareImages configurable by environment variable. Please read directions in conftest.py. Default behaviour is to call pytest.skip anytime that CompareImages is called.

Pull Request Testing

• Describe testing already performed for these changes:
  
  Tested locally but running with the env var set, and unset.
• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

Running export METPLOTPY_COMPAREIMAGES=$true in bash will enable these tests to run. I'm running on Linux, so they all fail. Suggest testing these on a system where they are expected to pass, to check we are getting the right results.

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

• Do these changes include sufficient testing updates? [Yes or No]
  yes

• Will this PR result in changes to the test suite? [Yes or No]
  
  If yes, describe the new output and/or changes to the existing output:
  
  Yes, changed behaviour in how tests are skipped.

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

• Please complete this pull request review by [Fill in date].
  

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) and Development issue
  Select: Milestone as the version that will include these changes
  Select: Coordinated METplus-X.Y Support project for bugfix releases or METplotpy-X.Y.Z Development project for official releases
• After submitting the PR, select the ⚙️ 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.
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