Add iterative training API

[mdekstrand]

This adds an IterativeTraining interface adding standardized iterative training support, and updates ALS to use it. Closes #438.

[codecov]

Codecov Report

Attention: Patch coverage is 98.41270% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.98%. Comparing base (1954efb) to head (2eba78d).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/training.py	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #601      +/-   ##
==========================================
+ Coverage   89.96%   89.98%   +0.01%     
==========================================
  Files         107      107              
  Lines        6587     6608      +21     
==========================================
+ Hits         5926     5946      +20     
- Misses        661      662       +1     

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