Demonstrate annotating structured metadata to classification functions

[cthoyt]

Summary

This PR demonstrates a potential solution to #28 in a pilot applied to classification metrics. It could be extended in a future pull request to other branches of the package.

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

Changes

• [x] Add an annotate decorator that adds some structured information to classification functions
• [x] Add tests to make sure the data is accessible
• [x] Add unit test to ensure all classification functions are annotated (1afd68b)
• [x] Annotate all classification functions

Future

Before finalizing this PR, I had also used the annotate function to make a registry of functions. This could be used to make the generation/maintenance of the ClassificationMetricSet much easier, but I'd save that for a different PR.

[codecov-commenter]

Codecov Report

Merging #29 (eaf835d) into main (e3db3f6) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   99.74%   99.76%   +0.01%     
==========================================
  Files          16       16              
  Lines         781      847      +66     
==========================================
+ Hits          779      845      +66     
  Misses          2        2              

Impacted Files	Coverage Δ
rexmex/metrics/classification.py	100.00% <100.00%> (ø)
rexmex/utils.py	100.00% <100.00%> (ø)
tests/unit/test_metrics.py	100.00% <100.00%> (ø)

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[benedekrozemberczki]

Thank you @cthoyt!