getNodesInCluster(clusterId) {
    const nodesArray = [];
    if (this.isCluster(clusterId) === true) {
      const containedNodes = this.body.nodes[clusterId].containedNodes;
      for (const nodeId in containedNodes) {
        if (Object.prototype.hasOwnProperty.call(containedNodes, nodeId)) {
          nodesArray.push(this.body.nodes[nodeId].id);
        }
      }
    }

    return nodesArray;
  }
@param {Cluster.id} clusterId
@returns {Array.<Node.id>}