Clean up ALS and fix RR1 bug

[mdekstrand]

This cleans up ALS a bit, and fixes a subtle bug in the coordinate descent implementation.

[codecov]

Codecov Report

Merging #231 (76e7778) into series/0.12 (9b481d6) will decrease coverage by 23.69%.
The diff coverage is 33.33%.

@@               Coverage Diff                @@
##           series/0.12     #231       +/-   ##
================================================
- Coverage        92.05%   68.36%   -23.70%     
================================================
  Files               43       43               
  Lines             3473     3464        -9     
================================================
- Hits              3197     2368      -829     
- Misses             276     1096      +820     

Impacted Files	Coverage Δ
lenskit/algorithms/als.py	53.16% <33.33%> (-44.60%)	⬇️
lenskit/util/accum.py	22.22% <0.00%> (-77.78%)	⬇️
lenskit/algorithms/svd.py	30.15% <0.00%> (-65.08%)	⬇️
lenskit/algorithms/implicit.py	27.53% <0.00%> (-55.08%)	⬇️
lenskit/crossfold.py	46.28% <0.00%> (-52.90%)	⬇️
lenskit/util/array.py	50.00% <0.00%> (-50.00%)	⬇️
lenskit/util/test.py	41.66% <0.00%> (-45.84%)	⬇️
lenskit/util/data.py	53.33% <0.00%> (-40.01%)	⬇️
lenskit/math/solve.py	52.63% <0.00%> (-36.85%)	⬇️
... and 23 more

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