Create a unique key for a group of layers
@param {Object[]} layer_group An array of Mapbox GL layers.

@returns {string} The layer_group source name, followed by a concatenated string of layer ids.
function getLayerGroupName(layer_group) {
  const all_names = [];
  for (let i = 0, ii = layer_group.length; i < ii; i++) {
    all_names.push(layer_group[i].id);
  }
  return `${layer_group[0].source}-${all_names.join(',')}`;
}