Option to allow `breadth_first_search` to use `in_edges`

[byjtew]

Context: You want to create a predecessors(source_node) function gathering the vertices that point to a given source_vertex, you'd want to use a BFS visitor, which is currently impossible since the usage of the out_edges iterator is hardcoded in its behaviour.

Feature request: A new function inv_breadth_first_search, or an extra template parameter to specify which iterator to use (in_edges/out_edges)

Here's the local version that we use:

template<class IncidenceGraph, class Buffer, class BFSVisitor, class ColorMap, class SourceIterator>
void inv_breadth_first_visit(const IncidenceGraph &g, SourceIterator sources_begin, SourceIterator sources_end,
                             Buffer &Q, BFSVisitor vis, ColorMap color) {
    BOOST_CONCEPT_ASSERT((IncidenceGraphConcept<IncidenceGraph>));
    typedef graph_traits<IncidenceGraph> GTraits;
    typedef typename graph_traits<IncidenceGraph>::vertex_descriptor Vertex;
    BOOST_CONCEPT_ASSERT((BFSVisitorConcept<BFSVisitor, IncidenceGraph>));
    BOOST_CONCEPT_ASSERT((ReadWritePropertyMapConcept<ColorMap, Vertex>));
    typedef typename property_traits<ColorMap>::value_type ColorValue;
    typedef color_traits<ColorValue> Color;
    typename GTraits::in_edge_iterator ei, ei_end;

    for (; sources_begin != sources_end; ++sources_begin) {
        Vertex s = *sources_begin;
        put(color, s, Color::gray());
        vis.discover_vertex(s, g);
        Q.push(s);
    }
    while (!Q.empty()) {
        Vertex u = Q.top();
        Q.pop();
        vis.examine_vertex(u, g);
        for (boost::tie(ei, ei_end) = in_edges(u, g); ei != ei_end; ++ei) {
            Vertex v = target(*ei, g);
            vis.examine_edge(*ei, g);
            ColorValue v_color = get(color, v);
            if (v_color == Color::white()) {
                vis.tree_edge(*ei, g);
                put(color, v, Color::gray());
                vis.discover_vertex(v, g);
                Q.push(v);
            } else {
                vis.non_tree_edge(*ei, g);
                if (v_color == Color::gray())
                    vis.gray_target(*ei, g);
                else
                    vis.black_target(*ei, g);
            }
        } // end for
        put(color, u, Color::black());
        vis.finish_vertex(u, g);
    } // end while
}

cc @raphael-s-steiner