function TileAvailability(tilingScheme, maximumLevel) {
  this._tilingScheme = tilingScheme;
  this._maximumLevel = maximumLevel;

  this._rootNodes = [];
}
Reports the availability of tiles in a {@link TilingScheme}.

@alias TileAvailability
@constructor

@param {TilingScheme} tilingScheme The tiling scheme in which to report availability.
@param {number} maximumLevel The maximum tile level that is potentially available.