Added missing functions to ConstantSpline

[lilleyse]

Fixes #6416

To do:

• Test - need a model that has one keyframe.
• CHANGES.md

[cesium-concierge]

@lilleyse, thanks for the pull request! Maintainers, we have a signed CLA from @lilleyse, so you can review this at any time.

⚠️ I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

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[emackey]

CHANGES.md?

[lilleyse]

@emackey Updated.

[emackey]

Thanks @lilleyse.