The height of a tree 
is defined as the vertex height
of its root vertex, where the vertex
height of a vertex 
in a tree 
is the number of edges on the longest downward path between
and a tree leaf.
A function to return the height of a vertex 
in a tree 
may be implemented in a future version of the Wolfram
Language as TreeHeight[g, v], and a function to return
the height of a tree as TreeHeight[g].
