Add support for diagonal asymptotics in non-smooth cases

[turnip314]

No description provided.

[behackl]

The comments so far are mostly about documentation. I still need to review the "interesting" part of the code in asymptotics.py and whitney.py in detail; more comments following later. Looking good so far!

[behackl]

I've pushed some (hopefully) uncontroversial improvements in 7338dbe, 72121a7, 80f5ed8, b0ae16d -- and am now done with this branch.

There are two larger suggestions that I think would make sense to include, namely:

• refactoring diagonal_asy_smooth and turning it into a private function (-> _diagonal_asy_smooth) while moving the doctests to the new and improved diagonal_asy. I'd be more comfortable to modify the interface some more, which would allow reusing more code from diagonal_asy (like casting r, the rational splitting, ...) -- so effectively, to call _diagonal_asy_smooth a bit later.
• And second, I feel it would also be a good time to remove the deprecated as_symbolic keyword argument.

If you agree, I'd take care of both of these in follow-up PRs though, as I'd really like to get this merged now.

Pretty cool stuff! 🚀