Add sustainability score for simplified understanding through single metric

[d33bs]

Description

This PR adds a "sustainability score" metric which summarizes the findings of other metrics in one single value. This score is calculated using boolean metrics and a "sustainability direction" value per metric. The values are summed and divided by the number of metrics to then provide a score.

Along the journey towards this PR I noticed and fixed the following:

• Metrics types and type check tests for the metrics table (which presented challenges earlier and benefited the work in this PR).

Closes #135
Closes #207

What is the nature of your change?

• Content additions or updates (adds or updates content)
• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (these changes would cause existing functionality to not work as expected).

Checklist

Please ensure that all boxes are checked before indicating that this pull request is ready for review.

• I have read the CONTRIBUTING.md guidelines.
• My code follows the style guidelines of this project.
• I have performed a self-review of my own contributions.
• I have commented my content, particularly in hard-to-understand areas.
• I have made corresponding changes to related documentation (outside of book content).
• My changes generate no new warnings.
• New and existing tests pass locally with my changes.
• I have added tests that prove my additions are effective or that my feature works.
• I have deleted all non-relevant text in this pull request template.

[gwaybio]

Several discussion points this time