`copy` does not work with type TikzDocument

[rs7q5]

For some reason copy does not work with type TikzDocument, but does for something like TikzPicture and AxisLike objects. I think it is because the other axis elements are of OptionType. But was wondering if we could add a method so TikzDocument could be copied.

Edit: doing Base(td::TikzDocument) = deepcopy(td) seems to work. Let me know if this would be acceptable and I can cut a PR.