A tile in a 
Cesium3DTileset.  When a tile is first created, its content is not loaded;
the content is loaded on-demand when needed based on the view.
Do not construct this directly, instead access tiles through Cesium3DTileset#tileVisible.
Members
readonlyboundingSphere : BoundingSphere
    Get the bounding sphere derived from the tile's bounding volume.
readonlychildren : Array.<Cesium3DTile>
    Gets the tile's children.
readonlycomputedTransform : Matrix4
    The final computed transform of this tile.
readonlycontent : Cesium3DTileContent
    The tile's content.  This represents the actual tile's payload,
not the content's metadata in the tileset JSON file.
expireDate : JulianDate
    The date when the content expires and new content is requested.
    The time in seconds after the tile's content is ready when the content expires and new content is requested.
    Returns the 
        
            
extras property in the tileset JSON for this tile, which contains application specific metadata.
Returns undefined if extras does not exist.
    The error, in meters, introduced if this tile is rendered and its children are not.
This is used to compute screen space error, i.e., the error measured in pixels.
readonlyparent : Cesium3DTile
    This tile's parent or 
undefined if this tile is the root.
When a tile's content points to an external tileset JSON file, the external tileset's
root tile's parent is not undefined; instead, the parent references
the tile (with its content pointing to an external tileset JSON file) as if the two tilesets were merged.
readonlytileset : Cesium3DTileset
    The tileset containing this tile.
transform : Matrix4
    The local transform of this tile.
