Uses of colour refinement

[mtorpey]

Now that @RheyaM has implemented colour refinement (#905) we should think about what other things it could be useful for.

I'm making this issue as a place we can keep notes on its uses and begin to implement those. Would anyone like to leave some comments with those?

[flsmith]

The most immediate uses off the top of my head are:

1. Testing isomorphism of digraphs. This is currently handed off to bliss/nauty, which will do an optimised colour refinement itself but requires rebuilding the graph in bliss. There are probably some situations where it's better to try colour refinement in GAP before handing the graph over.
2. Testing vertex transitivity. Again, currently decided by checking if the automorphism group (from bliss/nauty) is transitive. More than one colour class means not vertex transitive, since the orbits of the automorphism group are contained within colour classes.
3. Quickly generating a supergroup of the automorphism group (maybe useful if you just need a bound)