PreImagesRepresentativeNC plan

[ChrisJefferson]

This is a PR to discuss a plan for converting PreImagesRepresentative to PreImagesRepresentativeNC, which would follow on from #5073 (there are a few methods being considered, but I'll just refer to PreImagesRepresentative here).

I don't have a plan, mainly just a list of comments :)

At present packages install PreImagesRepresentative, but in many cases the function they are installing needs renaming to PreImagesRepresentativeNC. While doing this, those packages may want to write a "proper" PreImagesRepresentative.

The question is, how to handle the "partially converted" world, where some packages have added PreImagesRepresentativeNC and a "proper" PreImagesRepresentative, and other packages have an old PreImagesRepresentative which is equivalent to PreImagesRepresentativeNC and no PreImagesRepresentativeNC?

I would be happy to put some thought into this question, but I don't have an immediate and good answer.