Add spread/skill ratio statistic

[dallasfoster]

Earth2Studio Pull Request

Description

Adds spread/skill ratio statistic Improves statistics test coverage

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [x] The CHANGELOG.md is up to date with these changes.
• [ ] An issue is linked to this pull request.

[dallasfoster]

/blossom-ci

[dallasfoster]

/blossom-ci

[dallasfoster]

/blossom-ci