Reintroduce check order of the k-component candidate after clique heuristic.

This aviods a possible division by zero if the biconnected component of the result
of the clique heuristic is not a k-core in the original graph. It is the last
sanity check that the algorithm does before considering a candidate to be a
k-component. This error was introduced while refactoring.

Also fixed error in reporting results for k = {1, 2}, isolated dyads have to have
connectivity 1, and isolated nodes connectivity 0. The error was also introduced
while refactoring.
Improve variable names and simplify parameters of clique_heuristic.

Adding comments explaining the algorithm still pending.
Improve antichains implementation.
[WIP] Fix k_cutset implementation, so far with an (ugly) hack.

Commenting the initial checks for biconnectedness (which return
articulation points insted of running the algorithm) revealed
a problem with the implementation. It was returning some incorrect
node cutsets, in addition of the correct cutsets. Now we only keep
looking for cuts in the condensation graph of the residual network
until we have k cuts. This shoud not be necessary, but I'm still
not sure if I got the whole antichans and closed sets right.

This pass all tests without the biconnectivity shortcut, also added
some new ones. The implementation has to be improved, now the auxiliary
data structures (residual network and auxiliary digraph) are not reused.