Support for entities as reference frames

[speigg]

Based on previous pull request #2998, with optimizations to eliminate allocations.

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This PR is based on the referenceFrames branch, and adds support for constructing trees of entities by allowing entities to be used as reference frames themselves (see #1045). The root reference frame for any tree is either FIXED, INERTIAL, or an entity with an undefined position (an arbitrary frame).

For example:

var e1 = new Entity();
e1.position = new ConstantPositionProperty(new Cartesian3(100000, 200000, 300000)); // implicit FIXED frame
var orientation = new Quaternion(0, 0, 1, 1);
e1.orientation = new ConstantProperty(Quaternion.normalize(orientation, orientation));

var e2 = new Entity();
e2.position = new ConstantPositionProperty(new Cartesian3(200, 400, 600), e1); // note: e1 here
orientation = new Quaternion(1, 0, 0, 1);
e2.orientation = new ConstantProperty(Quaternion.normalize(orientation, orientation));

var value = new Cartesian3(1, 2, 3);
var result = PositionProperty.convertToReferenceFrame(time, value, e2, ReferenceFrame.FIXED);

expect(result).toEqual(new Cartesian3(99603, 200201, 300602));

// Can also pass an arbitrary Entity as the referenceFrame parameter in getValueInReferenceFrame().
// The reference frame can be any Entity... if both entities are not connected, the result is undefined. 
e1.position.getValueInReferenceFrame(time, e2) 

PositionProperty.convertToReferenceFrame has been rewritten to support converting positions between any two reference frames, including any two entities. Likewise, OrientationProperty.convertToReferenceFrame has been added to convert orientations between any two reference frames.

ReferenceEntity class is added to support the '#id' notation for entities as reference frames in CzmlDataSource when the specified Entity does not yet exist in the EntityCollection (https://github.com/AnalyticalGraphicsInc/cesium/wiki/CZML-Content#position).

Specs have been added for PositionProperty and OrientationProperty.
JSHint passes.

Summary of Changes:

• Modified CzmlDataSource to process ‘#id’
• Added OrientationProperty (convertToReferenceFrame)
• Modified PositionProperty (convertToReferenceFrame)
• Added ReferenceEntity
• Added OrientationPropertySpec
• Fixed PositionPropertySpec tests in the referenceFrames branch
• Added OrientationPropertySpec

Features not included:

• getValueInReferenceFrame does not yet exist for the orientation property on the Entity class (the more cumbersome OrientationProperty.convertToReferenceFrame has to be used to convert orientations between reference frames instead). It seems that having a getValueInReferenceFrame method for orientation properties would require adding many new *OrientationProperty classes, the equivalent of all the *PositionProperty classes.
• The ability to specify a reference frame for an orientation other than the reference frame specified by the position property.

[cesium-concierge]

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

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[ggetz]

Thanks for the pull request @speigg !

@shunter Do you want to take a first look at this?

[cesium-concierge]

Thanks again for your contribution @speigg!

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[hpinkos]

@shunter @mramato what's the plan for this PR? Is this a change we'd like to take or do you think this should be closed?

[shunter]

Adding this capability is definitely desired. This has been on my list to look at but I don't know when I'll get to it. We need to ensure that it's added in way that enables it to work seamlessly and correctly across our product lines (Components/STK Desktop -> CZML -> Cesium).

When I have availability I expect I'll pick up from where this PR currently stands.

[cesium-concierge]

Thanks again for your contribution @speigg!

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---

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🌍 🌎 🌏

[cesium-concierge]

Thanks again for your contribution @speigg!

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[cesium-concierge]

Thanks again for your contribution @speigg!

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Thanks again for your contribution @speigg!

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[hpinkos]

@shunter any idea when you might be able to work on this? Should we park this on a branch?

[cesium-concierge]

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[hpinkos]

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[devond5]

So will there be a convertToReferenceFrame() method for orientation?? or only for position??

[ggetz]

I'm closing this for now as it's quite out of date with main. If we wish to revisit this in the future, let's open a fresh PR. Thanks all.