Add support for using a velocity vector as the aligned axis of a billboard in CZML

[shunter]

Introduce a new CZML property velocityReference which indicates that a value is determined by the velocity of an object's position, expressed using reference syntax. The reference can be to any object.

additionally:

• I clarified in several places that aligned axis must be a unit vector.
• As discussed offline, I made the vector normalization done by VelocityVectorProperty optional (default remains as before).

Specifically, consider whether this name is what we want. Given that it becomes part of the CZML spec, decisions are fairly permanent here.

"billboard" : {
    "alignedAxis" : {
        "velocityReference" : "#position"
    }
}

For this PR, there is a lot of churn in the validation document. My initial simplistic approach to producing unique packet IDs and unique values etc. caused everything to get re-numbered throughout the document. I have switched it to scope unique values a bit better from now on, so that adding a new property (e.g. velocityReference) won't affect quite as much of the document in the future.

[pjcozzi]

Who is the right person to review this?

Perhaps @tfili?

[mramato]

@shunter please update CHANGES.md

[mramato]

Other than that, this is fine with me. Can you merge in master, it looks like there are some changes part of this PR that I think should already be merged (czml validation stuff)

[shunter]

Updated.

[mramato]

Thanks.