Use registry pattern for ClassificationMetricSet

[cthoyt]

Summary

This PR upgrades the rexmex.utils.Annotator class to implement the registry pattern, which basically means it keeps track of all of the functions it gets applied to. This is actually the same thing that the ClassificationMetricSet class does in its __init__, so it can be used to reduce the hard-coded aspects there.

Previously existing tests should be sufficient to check that this still has the same functionality as before (with the added benefit of including all metrics instead of just a handful)

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

Changes

• Implement registry pattern in rexmex.utils.Annotator
• Refactor rexmex.metricset.ClassificationMetricSet to use registry from rexmex.metrics.classification.classifications

[codecov-commenter]

Codecov Report

Merging #40 (b62e0ed) into main (d18c846) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #40   +/-   ##
=======================================
  Coverage   99.77%   99.77%           
=======================================
  Files          16       16           
  Lines         884      886    +2     
=======================================
+ Hits          882      884    +2     
  Misses          2        2           

Impacted Files	Coverage Δ
rexmex/metricset.py	98.24% <100.00%> (-0.09%)	⬇️
rexmex/utils.py	100.00% <100.00%> (ø)

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