[ENH:] Cumulative Gain Chart: Allow single line version

[emrynHofmannElephant]

Just wondering if there could be a small change to the function here:

sklearn-evaluation/src/sklearn_evaluation/plot/cumulative_gain_lift_curve.py

Line 99 in 96456ee

def cumulative_gain(

So that it allows for one set of predicted probabilities (instead of requiring two). In particular this would require a change from y_score to y_pred, and likely either the removal of the second call to _cumulative_gain_curve() on line 164 & 171.

As this function is limited to the binary case anyway, the second line is redundant, as it is just the inverse probability of the other. Therefore, in case both lines are needed, a simple input check (y_score.shape[1]) & calculation could generate the second set instead?