Add a popularity metric

[mdekstrand]

This adds the MeanPopRank metric for computing the popularity or obscurity of recommended items.

[codecov]

Codecov Report

Attention: Patch coverage is 95.34884% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.25%. Comparing base (d6e8c80) to head (aab732f).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/metrics/ranking/_pop.py	93.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #622      +/-   ##
==========================================
+ Coverage   90.23%   90.25%   +0.02%     
==========================================
  Files         111      112       +1     
  Lines        7769     7809      +40     
==========================================
+ Hits         7010     7048      +38     
- Misses        759      761       +2     

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