code: def node_scale_root_height(self, treeheight=1):
          
          # make tree height = 1 * treeheight
          ctree = self._ttree.copy()
          _height = ctree.treenode.height
          for node in ctree.treenode.traverse():
              node.dist = (node.dist / _height) * treeheight
          ctree._coords.update()
          return ctree
docs: Returns a toytree copy with all nodes scaled so that the root 
          height equals the value entered for treeheight.