Adjust file types to be a set, making the inclusion of 'group' types trivial

[r-sharp]

PR Summary

Sci/Tech Reviewer: @Pierre-siddall
Code Reviewer:

Subtle shit in the options for "file types" to store as a set which makes setting up "group" file -types, i.e. ones which resolve into multiple others, trivial.

I've added a demo one called CI, which will currently check Fortran and Python files.

Code Quality Checklist

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid understanding and enhance the readability of the code
• My changes generate no new warnings
• All automated checks in the CI pipeline have completed successfully

Testing

• This change has been tested appropriately (please describe)

I ran the umdp3 checker on it's containing branch.
The group filetype 'CI' corectly resolved to check "Fortran, Python" files
running pytest on the script_umdp3_checker directory returns no failures.

Security Considerations

• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable)
• Authentication and authorisation are properly implemented (if applicable)

AI Assistance and Attribution

• Some of the content of this change has been produced with the assistance of Generative AI tool name (e.g., Met Office Github Copilot Enterprise, Github Copilot Personal, ChatGPT GPT-4, etc) and I have followed the Simulation Systems AI policy (including attribution labels)

GitHub CoPilot is turned on for "command completeion" when editing files. Some of the things it suggested were close enough to what was in my head that I accepted them.

Sci/Tech Review

• I understand this area of code and the changes being added
• The proposed changes correspond to the pull request description
• Documentation is sufficient (do documentation papers need updating)
• Sufficient testing has been completed

(Please alert the code reviewer via a tag when you have approved the SR)

Code Review

• All dependencies have been resolved
• Related Issues have been properly linked and addressed
• Code quality standards have been met
• Tests are adequate and have passed
• Security considerations have been addressed
• Performance impact is acceptable