Feature #461 hovmoeller tests

[John-Sharples]

NOTE - this is built on top of #465. This PR should go in after, or instead of, #465.

In this PR

• conftest function to create temporary netCDF file
• new tests for hovmoeller plots
• reinstate broken functionality for writing html for hovmoeller plots
• some tidy up of scatter plot tests

Pull Request Testing

• Describe testing already performed for these changes:
  
  Tests run

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  None

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

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

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

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[bikegeek]

This version of the scatter plot is no longer useful, it has been refactored, using matplotlib and is pending further instructions. Work is in the feature_23_scatter feature branch, and user documentation on read the docs: https://metplotpy.readthedocs.io/en/feature_23_scatter/Users_Guide/scatter.html

The new scatter plot's imports are consistent with the suggested changes

[bikegeek]

this won't be necessary, a new config file corresponding to the refactored scatter plot will replace this config file

[bikegeek]

The scatter plot changes aren't necessary, as the scatter plot will be replaced by the refactored version in feature_23_scatter