setDefaultNodeLabel(newDefault) {
    this._defaultNodeLabelFn = newDefault;
    if (typeof newDefault !== 'function') {
      this._defaultNodeLabelFn = () => newDefault;
    }

    return this;
  }
Sets the default node label. If newDefault is a function, it will be
invoked ach time when setting a label for a node. Otherwise, this label
will be assigned as default label in case if no label was specified while
setting a node.
Complexity: O(1).