[WIP] Add spatial index container

[jenningsanderson]

Category

What kind of change is this?

Please select one of the following five options.

Consult Pull request merging criteria for a description of each category.

1. MAJOR schema change as defined in Schema versioning and stability.
2. MINOR schema change as defined in Schema versioning and stability.
3. Cosmetic change.
4. Documentation change by member.
5. Documentation change by Overture tech writer.

Major change release plan

TODO: For any non-MAJOR change, delete this whole section.

For a MAJOR change as defined in Schema versioning and stability,
indicate the expected release date, related minor change steps, and your
public documentation and messaging plan.

A. Expected release date for this MAJOR change

TODO.

B. Related MINOR change steps

• TODO. List each related MINOR change as a bullet.

C. Public documentation and messaging lan

TODO.

Description

Brief description of the business purpose and effect of the pull request.

TODO.

Reference

List of relevant links to GitHub issues, PRs, and other documentation.

1. TODO.

Testing

Brief description of the testing done for this change showing why you are confident it works as expected and does not introduce regressions. Provide sample output data where appropriate.

TODO.

Checklist

Checklist of tasks commonly-associated with schema pull requests. Please review the relevant checklists and ensure you do all the tasks that are required for the change you made.

1. Add relevant examples.
2. Add relevant counterexamples.
3. Update any counterexamples that became obsolete. For example, if a counterexample uses property A but is not intended to test property A's validity, and you made a schema change that invalidates property A in that counterexample, fix the counterexample to align it with your schema change.
4. Update in-schema documentation using plain English written in complete sentences, if an update is required.
5. Update Docusaurus documentation, if an update is required.
6. Review change with Overture technical writer to ensure any advanced documentation needs will be taken care of, unless the change is trivial and would not affect the documentation.

Documentation Website

Update the hyperlink below to put the pull request number in.

[Docs preview for this PR.](https://dfhx9f55j8eg5.cloudfront.net/pr/<PUT THE PR # HERE>)