CZML and Entity articulations API

[emackey]

Support for articulations was added at the graphics primitive level in the 1.58 release (in #7835).

This PR adds support at the entity and CZML layers. The implementation is very similar to how the existing nodeTransformations work at those layers.

[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.

Reviewers, don't forget to make sure that:

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

[emackey]

Some sample articulations CZML is used in the new Sandcastle Demo.

    "model": {
        "gltf" : "https://assets.agi.com/models/launchvehicle.glb",
        "scale" : 2.0,
        "minimumPixelSize" : 128,
        "runAnimations" : false,
        "articulations" : {
            "Fairing Open" : {
                "epoch" : "2019-06-01T16:00:00Z",
                "number" : [
                    0, 0,
                    600, 120
                ]
            },
            "Fairing Separate" : {
                "epoch" : "2019-06-01T16:00:00Z",
                "number" : [
                    0, 0,
                    400, -50
                ]
            },
            "Fairing Drop" : {
                "epoch" : "2019-06-01T16:00:00Z",
                "interpolationAlgorithm" : "LAGRANGE",
                "interpolationDegree" : 2,
                "number" : [
                    0, 0,
                    80, 0,
                    100, 0,
                    120, -1,
                    600, -120
                ]
            }
        }
    }

[emackey]

Apparently Travis never got the message to GitHub about completion of the most recent build.

[shunter]

Could use some tests in CzmlDataSourceSpec (patterned off the tests for nodeTransformations) Otherwise this all looks good. I'll double check that this syntax works out for czml-writer.

[shunter]

I was able to get the writer working with a value directly under a property bag rather than the nodeTransformations which had an extra level.

Syntax in C# looks like:

using (var articulationsWriter = modelWriter.OpenArticulationsProperty())
using (var articulationWriter = articulationsWriter.OpenArticulationProperty("articulation1 stage1"))
{
	articulationWriter.WriteNumber(3.5);
}

So this all should work out.

[emackey]

@shunter I added some tests to CzmlDataSourceSpec. I didn't attempt to regenerate the CZML ValidationDocument, can you do that?

[shunter]

Yeah I can update the validation document separately. My local czml-writer changes are dependent on the changes made for #7899 so once that's merged I can add articulations on top of that.