getLayerOpacity (layerName) {
    const layer = this.layer_map[layerName]
    if (!layer) {
      return null
    }
    return layer.getOpacity()
  }
Returns the opacity of the given layer.  If the input name is not a layer, `null` is returned.
@param {string} layerName - name of the layer on which to get the opacity
@returns {?Float} The opacity value of the given layer.  This will be a value between 0.0 and 1.0, or `null`
if `layerName` is not a valid layer