function getUnitNeighborLayerIDs(neighbors) {
  let neighbor_layer_ids = neighbors.map(function (neighbor) {
    if (neighbor !== null) {
      let neighbor_layer_id = null;

      this.units.eachLayer(function (possible_neighbor_layer) {
        if (possible_neighbor_layer.feature.properties.id === neighbor) {
          neighbor_layer_id = this.units.getLayerId(possible_neighbor_layer);
        }
      }, this);

      return neighbor_layer_id;
    } else {
      return null;
    }
  }, this);

  return neighbor_layer_ids;
}
Given an array of pre-layer IDs, check if any of them correspond to the pre-layer IDs of unit layers, and if so
return an array of the corresponding layer IDs.
@private

@param {Array<?number>} - An array of pre-layer feature IDs for a unit's neighbors.
@returns {Array<?number>} - An array of Leaflet layer IDs corresponding to the unit's neighbors.