Introduce supplementary datums

[Jimbo4350]

Changelog

- description: |
    Introduce supplementary datums
  type:
  - feature        # introduces a new feature

Context

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How to trust this PR

Highlight important bits of the PR that will make the review faster. If there are commands the reviewer can run to observe the new behavior, describe them.

Checklist

• [ ] Commit sequence broadly makes sense and commits have useful messages
• [ ] New tests are added if needed and existing tests are updated. See Running tests for more details
• [ ] Self-reviewed the diff

[smelc]

cc @mmontin: is this something you would benefit from?