Use knowledge of cographs to make things faster

[mtorpey]

@coraaked is in the process of contributing an IsCograph function that will tell us whether a graph is a cograph or not.

In her presentation she told us some interesting stuff about trees and looking properties up, with alternative representations of cographs as trees with 1s and 0s on the nodes.

This makes me think that, if we recognise that a graph is a cograph, we may be able to take advantage of this knowledge computationally. This would be in the form of:

• Immediate methods
• "Set" operations called before returning from IsCograph
• Methods that take IsCograph as a filter and do something fast that relies on cographness.

Someone should look into what mathematical facts we can say about cographs, and translate these into code where appropriate.