Add / Modify biconnected components

[akcube]

The biconnected components template is pretty neat to quickly get the components, but constructing the block-cut tree isn't very trivial using them. Maybe add some text in notebook / add code to construct the relevant trees using BiconnectedComponents.h.

The callback passed to bicomps to get the bcc's is very un-intuitive. Add a small compute function to generate it from an edgelist (it's in the comments, move it to code).

int eid = 0; ed.resize(N); // global adj list, bcc uses. Maybe move to namespace / modify flow?
for each edge (a,b) {
    ed[a].emplace_back(b, eid); ed[b].emplace_back(a, eid++); 
}
bicomps([\&](const vi\& edgelist) {...}); // edgelist contains the *edges* in each biconnected component.