Implement `QuotientsDigraph`

[james-d-mitchell]

As the title says, it'd be good to have a function that computed all the quotients of a given digraph. I'm not sure how feasible this is really, but it'd be great to have something that worked for small examples.

[mpan322]

Meeting Disucssion:

• Want to get a list where every quotient digraph in it is unique.
• Number possible partitions is the bell number of number of verticies i.e. super inefficient to check them all.
• Potentially backtracking search using the stabalizer of the set. (out of my knowledge - somone who knows please add clarification for this)