Implement constraints on predicted prevalence (demographic parity)

[AndreFCruz]

Summary

We should enable creating group-wise constraints on the percentage of positive predictions (a.k.a., predicted prevalence).

This enables the popular Demographic Parity fairness metric.

TODO

• Add constraint_type=PP to the available group-wise constraint options;
• Implement gradient descent on proxy predicted prevalence metrics;
• Add the same PP constraint type for global constraints
  • This would be a constraint on maximum alert rate -> quite useful for our in-house use cases

EDIT

Minor notes for the future PP constraint implementation:
83de2426-ea6e-4308-9f0d-5ddc3b001580.pdf