1922 add metplus pointstat capability for met office

[RachelNorth]

Initial PointStat tests at the Met Office. Note that these are not final configurations.

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• Documentation has been updated to reflect change.
• New code has tests, and affected old tests have been updated.
• All tests and CI checks pass.
• Ensured the pull request title is descriptive.
• Conda lock files have been updated if dependencies have changed.
• Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• Marked the PR as ready to review.

[github-actions]

[JorgeBornemann]

All good. These are mostly METPlus configuration files that provide an initial capability and framework to support adding more statistics.

Note that the variables METPLUS_AREA_DIR and 'METPLUS_STN_DIR` are in the workflow optional configuration in the restricted file repository. They need to be included in other sites configurations.