Trac #17647: Branch and Bound for vertex separation

This patch implements a branch and bound algorithm for computing the
vertex separation of directed graphs. It can be significantly faster
than other methods already implemented. Furthermore, it can handle
graphs with more than 31 vertices, but of course the computation time
could be long.

{{{
sage: from sage.graphs.graph_decompositions import vertex_separation as
VS
sage: D = digraphs.RandomDirectedGNP(30,.1)
sage: %time w,l = VS.vertex_separation(D); print w
4
CPU times: user 229 ms, sys: 345 ms, total: 574 ms
Wall time: 575 ms
sage: %time w,l = VS.vertex_separation_BAB(D); print w
4
CPU times: user 1.34 ms, sys: 32 µs, total: 1.37 ms
Wall time: 1.35 ms
}}}

URL: http://trac.sagemath.org/17647
Reported by: dcoudert
Ticket author(s): David Coudert
Reviewer(s): Nathann Cohen