Retroactive union find

[alkhaled]

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I thought this project was pretty cool, so I took a stab at implementing fully retroactive union find using link cut trees as the backing data structure.

The semantics for updating and querying use time deltas to specify time, similar to the existing tminus semantics. The only difference is that is supports operations in the future as well as the past. You can find an example usage in examples.py

[csvoss]

Sorry for the delay here! I've realized recently that I've not been getting the GitHub notifications that I've wanted to get, so I just saw this.

At a glance, this looks really really nice! I appreciate the tests, and it fits well into the rest of the codebase.

I'll go through in more depth and give it a proper code review soon. Thanks for writing this!

[alkhaled]

No worries. Thanks for creating the project!