getNode(dataOrNode) {
    let node;

    if(this.nodes.has(dataOrNode)) {
      node = this.nodes.get(dataOrNode);
    } else if(isNode(dataOrNode)) {
      node = dataOrNode;
    } else {
      node = new Node(dataOrNode);
    }
    return node;
  }
Get existing node or create a new one
@param dataOrNode
@returns {*}