Document PolylineVolume

[thw0rted]

Cesium added support for PolylineVolume a few months ago, in CesiumGS/cesium#8841, but it's not listed on the wiki. As far as I can tell, that's still the authoritative place to go for CZML syntax, right?

[thw0rted]

Reading over the linked PR, this is my first pass at a Typescript interface, which should more or less map one-to-one with the docs format on the wiki:

export interface PolylineVolume {
    positions?: PositionList;
    shape?: PositionList;
    show?: boolean;
    cornerType?: {cornerType: CornerTypeValue} | ReferenceProperty;
    fill?: boolean;
    material?: Material;
    outline?: boolean;
    outlineColor?: ColorValue | ReferenceProperty;
    outlineWidth?: number;
    granularity?: number;
    shadows?: ShadowModeProperty | ReferenceProperty;
    distanceDisplayCondition?: DistanceDisplayConditionProperty | ReferenceProperty;
}

[astrojuanlu]

JSON schema lives here, and PolylineVolume is still not present: https://github.com/AnalyticalGraphicsInc/czml-writer/tree/master/Schema

[shunter]

Added in #183.