Allow -inf scores in ROC

[ego-thales]

Update code, docs and tests accordingly.

Decided in favor of adding $(0, 0)$ when necessary because of $-\infty$ score on negative sample. Correspond to the trivial "False" classifier and associated thresholds are [-inf, -inf[, with explanation in ROC.thresholds.

Closes #19.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9fff016) to head (b6f6653).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop       #41   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           69        69           
  Lines         3723      3723           
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  Hits          3723      3723           

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