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.

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
                ]
            }
        }
    }

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

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.

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.

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

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.