Issue #515 Implementing IsCograph Function

[coraaked]

Code that identifies, given a symmetric digraph input, whether or not the given graph is a cograph or not. It implements the algorithm given in "Habib, M & Paul, C & Viennot (2005), A Simple Linear Time Recognition Algorithm for Cographs", which breaks down the vertices into a permutation, and then identifies whether or not this vertex set is a factorising permutation.

This code is quite verbose, and could be improved to work more efficiently with larger cographs, as there are a lot of internal functions used bound to slow it down. I would be interested to do some more work on this, or if somebody else wants to have a look, to see if there are methods of improvement before the function is properly implemented.