Add CMIP7 future scenario solar variability files

[mo-jmanners]

PR Summary

Sci/Tech Reviewer: @martinbandrews
Code Reviewer: @r-sharp

Spectral variability files are added for the CMIP7 future solar scenario. The added files are documented in the README file for the directory. The new spectral_var 'scenario' files contain the historical data from 1850-2023 followed by the future scenario data from 2024-2299. They historical data in these files is identical to the 'ref' spectral_var files and can therefore be used in-place of those files with bit-comparable results.

An additional PI control file is added with the year changed from 1850 to 1. This will allow control runs to start from any year after year 1 using the mean PI control solar spectrum.

• linked Script to generate CMIP7 future scenario solar forcing for GA9 spectral files socrates#16
• closes CMIP7 future scenario solar forcing for GA9 spectral files #5

Code Quality Checklist

(Some checks are automatically carried out via the CI pipeline)

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid undertanding and enhance the
  readability of the code
• My changes generate no new warnings

Testing

• No shared files have been modified.

• If shared files have been modified, I have run the UM and LFRic Apps rose
  stem suites

• If any tests fail (rose-stem or CI) the reason is understood and
  acceptable (eg. kgo changes)

• I have added tests to cover new functionality as appropriate (eg. system
  tests, unit tests, etc.)

trac.log

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)

Performance Impact

• Performance of the code has been considered and, if applicable, suitable
  performance measurements have been conducted

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)

Documentation

• Where appropriate I have updated documentation related to this change and
  confirmed that it builds correctly

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
• CLA compliance has been confirmed
• Code quality standards have been met
• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable