def remove_vertex(vertex)
      @vertices[vertex].each do |edge|
        remove_edge(edge) if edge.include?(vertex)
      end
      delete_vertex(vertex)
      callback(vertex, :removed_from_graph, self)
    end
def remove_vertex(vertex)
      @vertices[vertex].each do |edge|
        remove_edge(edge) if edge.include?(vertex)
      end
      delete_vertex(vertex)
      callback(vertex, :removed_from_graph, self)
    end
Remove a vertex from the graph