Re-add unnormalized DCG metric

[mdekstrand]

This brings back the unnormalized DCG metric.

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.04%. Comparing base (33c5ec1) to head (7279628).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/metrics/ranking/_dcg.py	91.66%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #608   +/-   ##
=======================================
  Coverage   90.04%   90.04%           
=======================================
  Files         108      108           
  Lines        6790     6814   +24     
=======================================
+ Hits         6114     6136   +22     
- Misses        676      678    +2     

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