Implement diversity stats collector (collect_diversity_stats func)

[InnovativeInventor]

This is a draft PR which adds a collect_diversity_stats() function that reports the diversity of a given chain. I will come back and add docs/tests.

[codecov]

Codecov Report

Merging #400 (3710eed) into main (27cc3cd) will increase coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #400      +/-   ##
==========================================
+ Coverage   87.77%   87.97%   +0.20%     
==========================================
  Files          37       39       +2     
  Lines        1742     1772      +30     
==========================================
+ Hits         1529     1559      +30     
  Misses        213      213              

Impacted Files	Coverage Δ
gerrychain/meta/__init__.py	100.00% <100.00%> (ø)
gerrychain/meta/diversity.py	100.00% <100.00%> (ø)

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[InnovativeInventor]

I just added docs and tests. Should be ready for review now.

[chanelrichardson]

This is awesome! I've often wondered about this, given 100,000 steps I feel like it makes more sense to report the unique and accepted plans rather than making it seem like there's definitively 100,000 to report stats on.