setOpacity (opacity) {
    if (typeof opacity === 'number' && opacity >= 0.0 && opacity <= 1.0) {
      this.group_.setAttribute('opacity', opacity)
    }
  }
Sets the opacity of this layer. If opacity is not a value between 0.0 and 1.0,
nothing happens.
@param {Float} opacity - A float value in the range 0.0-1.0
@returns {void}