showOverlayByClass(layerClass) {
    if (layerClass != null && !(this._overlay.getCurrentLayer() instanceof layerClass)) {
      return this.showOverlay(new layerClass());
    }
    return this._overlayPromise;
  }
Shows overlay layer if it not already showing overlay of layerClass and returns a promise that resolves when the overlay has been fully swapped.
@param layerClass
@returns {Promise}