Initial support for AGI_articulations

[emackey]

This PR has initial support for 3D model articulations from the AGI_articulations vendor extension to glTF 2.0. There is no pointing vector or attach point support here, just articulations.

This isn't polished yet, but I'm opening it for early review from @shunter, @abwood, and @gbeatty.

TODO

• There should be a better way to expose the available articulation names, stage name, min, max, and get/set current stage value. Let's push this until a more complete solution for 3D Model Entity API at a disadvantage #7831 is designed.
• Add unit tests.
• Possibly in a separate PR, add support at the Entity layer. Also pushed.

[cesium-concierge]

Thanks for the pull request @emackey!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[emackey]

Here's a live demo.

[TJKoury]

This is awesome!

[emackey]

The scope of this PR has been dialed in a little. It's write-only (same as nodeTransformations) support for articulations at the model primitive level only.

Ready for full review & merge.

[emackey]

Any chance this can be merged in time for release? Scott doesn't know the contents of Model.js well enough to be comfortable merging this himself. We're confident in the articulations functionality, just need to make sure we're not breaking some conventions or established patterns in that file.

[lilleyse]

I took a quick look at the Model.js changes. They look totally fine to me. Should this change be mentioned in CHANGES.md?

[emackey]

CHANGES.md updated, thanks.

[emackey]

@shunter Can you do the honors, or does this need additional reviewers?

[abwood]

The application of articulation transformations looks to be consistent with STK's implementation. Here's the sand castle example using facility.glb:

and here are those same articulations applied to the default facility in STK:

[abwood]

Looks good on my end once the Travis CI issue is adjudicated.

[emackey]

Thanks! Travis is a fluke, some unrelated test flaked out.