Coverage refactor, added CoverageMetricSet and CoverageScoreCard

[kajocina]

Summary

Please provide a high-level summary of the changes for the changes and notes for the reviewers

• [X] Code passes all tests
• [X] Unit tests provided for these changes
• [X] Documentation and docstrings added for these changes

Changes

• changed signature for the Coverage metrics, it now requires supplying the relevant user and item spaces plus a list of tuples (user, item) as final predictions
• added the item coverage and user coverage metrics to CoverageMetricSet
• created a ScoreCard for coverage metrics (they need a different signature than classification and regression)

[cthoyt]

Would also be good to add an annotator for coverage functions, same as for classification functions. I could do this in a later PR :)

[codecov-commenter]

Codecov Report

Merging #42 (ee64141) into main (8fe5de1) will decrease coverage by 0.41%. The diff coverage is 93.05%.

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
- Coverage   99.77%   99.36%   -0.42%     
==========================================
  Files          16       16              
  Lines         886      942      +56     
==========================================
+ Hits          884      936      +52     
- Misses          2        6       +4     

Impacted Files	Coverage Δ
rexmex/metrics/coverage.py	81.81% <81.81%> (-5.69%)	:arrow_down:
rexmex/scorecard.py	97.82% <94.44%> (-2.18%)	:arrow_down:
rexmex/metricset.py	98.36% <100.00%> (+0.11%)	:arrow_up:
tests/integration/test_aggregation.py	100.00% <100.00%> (ø)
tests/unit/test_metrics.py	100.00% <100.00%> (ø)

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