CPO `vertex_value` -- what for?

[akrzemi1]

What is the purpose of the CPO vertex_value?

Graph containers customize it, but it is never used:

• neighbors.hpp and vertexlist.hpp mention it only in comments;
• there is one usage in bellman_ford_shortest_paths.hpp but it is clearly a bug: Fix Bellman with visitor #180.

[akrzemi1]

If it is used for anything, it requires some rethinking in branch desc4. The friend function in compressed_graph takes a vertex descriptor, while the CPO requires avertex reference.