Store colouring inside a digraph for isomorphism testing

[markusbaumeister]

Currently there is a method IsIsomorphicDigraph which takes two digraphs and (optionally) two colourings of them. This requires the user to store the digraph and the colour separately (or at least remember to take the colouring that might be stored in VertexLabels).

This is impractical at least in the following use-case: Assume there is an attribute which should be a coloured digraph. Since the vertex labels are usually used to suggest a connection between the graph and the original object they can't be used to store the colouring. While it would be possible to define an additional attribute describing the colouring, that is not something that a user expects (especially since the alternative package GRAPE can handle those colourings without issue).

The user would expect that they can just evaluate this attribute for different objects and then work with them. This includes testing for isomorphism. It is possible to define an IsIsomorphic-operation on the original object that computes the isomorphism with additional colours but this would again be unexpected and furthermore preclude the user from doing this on their own (which might be needed in their programs).

Therefore it would be helpful if there were an option to make ColouredDigraph or at least allow a colouring to be defined (which is not necessarily a proper vertex colouring) that is picked up in the IsIsomorphicDigraph-method.

[james-d-mitchell]

I am currently in the process of doing this.

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