velocityReference is not supported within an interval.

[shunter]

Attempting to define an alignedAxis as a velocityReference does not work within an interval, producing the error:

RuntimeError: {"interval":"2016-06-17T12:00:30Z/2016-06-17T12:01:00Z","velocityReference":"#position"} is not a valid CZML interval.

In CzmlDataSource, velocityReference is handled directly in processAlignedAxis, which precludes its use anywhere other than as a constant alignedAxis value.

unwrapCartesianInterval may be a better place to put the handling. This would merit some careful consideration though, as that would allow using velocityReference in any context that needs a Cartesian. For example you could use the velocity direction of an object as another object's position, which would not really make sense. Perhaps we should separate the concepts of a position vector (where magnitude is meaningful) from a direction vector (generally a unit vector). Currently we jumble it all together and assume everything's fine. (which is conventional in Cesium where Cartesian3 represents both kinds of vectors)

CZML:

[
    {
        "id": "document",
        "version": "1.0",
        "clock": {
            "interval": "2016-06-17T12:00:00Z/2016-06-17T12:01:00Z",
            "currentTime": "2016-06-17T12:00:00Z"
        }
    },
    {
        "position": {
            "epoch": "2016-06-17T12:00:00Z",
            "cartesian": [
                0,
                1216361.4096947117,
                -4736253.175342511,
                4081267.4865667094,
                60,
                1716361.4096947117,
                -4736253.175342511,
                4081267.4865667094
            ]
        },
        "billboard": {
            "image": "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsMAAA7DAcdvqGQAAACvSURBVDhPrZDRDcMgDAU9GqN0lIzijw6SUbJJygUeNQgSqepJTyHG91LVVpwDdfxM3T9TSl1EXZvDwii471fivK73cBFFQNTT/d2KoGpfGOpSIkhUpgUMxq9DFEsWv4IXhlyCnhBFnZcFEEuYqbiUlNwWgMTdrZ3JbQFoEVG53rd8ztG9aPJMnBUQf/VFraBJeWnLS0RfjbKyLJA8FkT5seDYS1Qwyv8t0B/5C2ZmH2/eTGNNBgMmAAAAAElFTkSuQmCC",
            "alignedAxis": [
                {
                    "interval": "2016-06-17T12:00:00Z/2016-06-17T12:00:30Z",
                    "unitCartesian": [
                        0,
                        1,
                        0
                    ]
                },
                {
                    "interval": "2016-06-17T12:00:30Z/2016-06-17T12:01:00Z",
                    "velocityReference": "#position"
                }
            ]
        }
    }
]