CZML Heading/Pitch/Roll orientaiton

[hpinkos]

It looks like CZML currently only supports orientation defined as a unit quaternion. We should add the ability to define orientation as a heading/pitch/roll.

Requested on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/4j991i8iazQ

[hpinkos]

I've been seeing this come up a few times lately on the forum so I added the priority label

[pjcozzi]

Can you add links to the other forum threads if there's others than the one linked above?

[mramato]

I wrote up AnalyticalGraphicsInc/czml-writer#133 in czml-writer, but we should also evaluate #5079 (or another form of it) before starting to make wider use of HPR. For example, Cesium and STK Components currently have different HPR conventions (I think Components is more typical here, but I'm really not sure).

[davidbuzz]

+1 for this.

[zhanglf1982]

+10000 for this.