addNeighbor(from, neighbor = {}) {
    for (let i = 0; i < this._nodes.length; i++) {
      if (this._nodes[i].id === from) {
        this._nodes[i].neighbors = this._nodes[i].neighbors || [];
        this._nodes[i].neighbors.push(neighbor);
        break;
      }
    }
  }
Add a neighbor to a node
@param {string} from
@param {object} neighbor