addRelation(parentNodeId, childNodeId) {
    if (this.childrenReference[parentNodeId] === undefined) {
      this.childrenReference[parentNodeId] = [];
    }
    this.childrenReference[parentNodeId].push(childNodeId);

    if (this.parentReference[childNodeId] === undefined) {
      this.parentReference[childNodeId] = [];
    }
    this.parentReference[childNodeId].push(parentNodeId);
  }
Add the relation between given nodes to the current state.
@param {Node.id} parentNodeId
@param {Node.id} childNodeId