setLayerVisibility (layerName, bVisible) {
    if (typeof bVisible !== 'boolean') {
      return null
    }
    const layer = this.layer_map[layerName]
    if (!layer) {
      return null
    }
    layer.setVisible(bVisible)
    return layer.getGroup()
  }
Sets the visibility of the layer. If the layer name is not valid, this
function returns `null`, otherwise it returns the `SVGElement` representing
the layer. This is an undo-able action.
@param {string} layerName - The name of the layer to change the visibility
@param {boolean} bVisible - Whether the layer should be visible
@returns {?SVGGElement} The SVGGElement representing the layer if the
  `layerName` was valid, otherwise `null`.