Summary

UnionFind is missing the ability to add new sets (increasing the internal vectors' length) after creating the UnionFind.

Motivation

It's part of the standard disjoint-set data-structure operations. Also, without it it makes it very annoying when trying to transform some problem into a disjoint-set data-structure, where you incrementally add sets and union existing sets as you go along, since currently the API limitations requires you to convert that incremental algorithm into a 2-pass algorithm so you can calculate the length to pass into UnionFind::new, and then the second pass actually uses the UnionFind data-structure.

Details

• What code needs to change? New traits? Which modules?

Add a new method to UnionFind to add a new set and return the new set's index.

• Is this a new graph algorithm? Include links to papers, Wikipedia, or other
  implementations of this algorithm that exist.

https://en.wikipedia.org/wiki/Disjoint-set_data_structure#Making_new_sets

• Are you willing to implement this yourself? Mentor someone else and help them
  implement it?

I can implement this, it's pretty trivial...