Remove CSR from LensKit deps

[mdekstrand]

This removes usage of the CSR package from LensKit, refactoring the sparse_ratings method to have a clearer API (closes #379).

[codecov]

Codecov Report

Attention: Patch coverage is 97.10145% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.96%. Comparing base (cbfe956) to head (19f933e).
Report is 620 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/data/matrix.py	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #421      +/-   ##
==========================================
+ Coverage   90.11%   90.96%   +0.84%     
==========================================
  Files          49       45       -4     
  Lines        3258     3077     -181     
==========================================
- Hits         2936     2799     -137     
+ Misses        322      278      -44     

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