getLayerVisibility (layerName) {
    const layer = this.layer_map[layerName]
    return layer ? layer.isVisible() : false
  }
Returns whether the layer is visible.  If the layer name is not valid,
then this function returns `false`.
@param {string} layerName - The name of the layer which you want to query.
@returns {boolean} The visibility state of the layer, or `false` if the layer name was invalid.