Add sparse matrix normalization routines + sparse util tests

[mdekstrand]

This makes a few sequenced changes to improve sparse matrix utilities and testing:

1. Add a sparse_tensors Hypothesis generator to lenskit.util.test.
2. Add tests for sparse_row_stats.
3. Add normalize_sparse_rows function to normalize rows of a CSR matrix, with tests.
4. Use normalize_sparse_rows in item KNN.

[codecov]

Codecov Report

Attention: Patch coverage is 91.22807% with 5 lines in your changes missing coverage. Please review.

Project coverage is 91.58%. Comparing base (2ed8cd5) to head (ffef967).
Report is 821 commits behind head on main.

Files	Patch %	Lines
lenskit/util/test.py	88.88%	3 Missing ⚠️
lenskit/data/matrix.py	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
- Coverage   91.64%   91.58%   -0.07%     
==========================================
  Files          48       48              
  Lines        3063     3100      +37     
==========================================
+ Hits         2807     2839      +32     
- Misses        256      261       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.