* inOrderTraversal(node = this.root) {
    if (node && node.left) { yield* this.inOrderTraversal(node.left); }
    yield node;
    if (node && node.right) { yield* this.inOrderTraversal(node.right); }
  }
In-order traversal on a tree: left-root-right.
If the tree is a BST, then the values will be sorted in ascendent order
@param {BinaryTreeNode} node first node to start the traversal
@yields {BinaryTreeNode}