A vertex mutable graph can be changed by adding or removing
vertices. The memory management is the responsibility of the graph
implementation. The graph user need only make calls to
add_vertex and remove_vertex and the graph
implementation does the rest.
Semantics: Remove u from the vertex set of the graph.
Preconditions:u is a valid vertex descriptor of graph g
and there are no edges incident to vertex u. The function
clear_vertex can be used to remove all incident edges.
Postconditions: num_vertices(g) is one less; u
no longer appears in the vertex set of the graph and it
is no longer a valid vertex descriptor.
Vertex insertion is guaranteed to be amortized constant time.