setLayerOpacity (layerName, opacity) {
    if (typeof opacity !== 'number' || opacity < 0.0 || opacity > 1.0) {
      return
    }
    const layer = this.layer_map[layerName]
    if (layer) {
      layer.setOpacity(opacity)
    }
  }
Sets the opacity of the given layer.  If the input name is not a layer,
nothing happens. If opacity is not a value between 0.0 and 1.0, then
nothing happens.
NOTE: this function exists solely to apply a highlighting/de-emphasis
effect to a layer. When it is possible for a user to affect the opacity
of a layer, we will need to allow this function to produce an undo-able
action.
@param {string} layerName - Name of the layer on which to set the opacity
@param {Float} opacity - A float value in the range 0.0-1.0
@returns {void}