Rename `Slices` to `SliceCategories` to avoid name conflict

[KevinDCarlson]

Julia complains all the time now about Catlab and Base both exporting the word Slices. Can we just change the name? This would be a breaking change, not sure whether there's a better approach but I think leaving it as is isn't a serious option in that the attempted export wouldn't actually work if you tried to use Slices unqualified.

[epatters]

Yes, we should change the name. I would suggest renaming the module to SliceCategories.

I'm not sure why this PR deletes the modules.

[KevinDCarlson]

I think I just didn't add the renamed files on the first commit?

[epatters]

Thanks! Can you please rebase and force-push so we get a clean commit?

[KevinDCarlson]

Yep, done.