ROC/AUC behaviour

[ritamtbsilva]

Currently ROC/AUC in markeR is always computed in module (absolute value), with automatic pairwise comparisons across levels. This forces all AUC ≥ 0.5, which is fine for discrimination but not for classification, where the direction matters and AUC < 0.5 is meaningful.

I propose adding:

• A parameter to choose between discrimination (current behaviour) and classification, with the latter as default;
• In classification mode, the user must specify positive (and optionally negative) class, along with the relevant variable.