several fixes and improvements to multigraph

[RazinShaikh]

These fixes were found while adding multigraph support in ZXLive. It would be nice to merge this sooner as the ZXLive main branch is broken right now.

[akissinger]

This is failing mypy because list.remove doesn't return a value in diff.py line 63, so I think it doesn't like you using it in a if statement. This could be what you expect, but I'm having a hard time reading this one-liner. Maybe try being a bit less fancy?

Also more generally, in places where we used sets before (e.g. when we want to check containment many times), I suggest we use a proper multiset implementation, like this one: https://docs.python.org/3/library/collections.html#collections.Counter rather than lists, otherwise we might start to run in to some performance problems.

[RazinShaikh]

That line is a bit hacky. I can change it to Counter.

[RazinShaikh]

I didn't test properly when I changed to Counter. I have fixed a bug where the correct way of looping through Counter is to call Counter.elements(). It is fixed in PR #241.