Internal use only. Adapted from Ferrier, Pearce and Watson's code, by J.Elith
, see: Hanley, J.A. & McNeil, B.J. (1982) The meaning and use of the area
under a Receiver Operating Characteristic (ROC) curve. Radiology, 143, 29-36.
Also Pearce, J. & Ferrier, S. (2000) Evaluating the predictive performance of
habitat models developed using logistic regression. Ecological Modelling,
133, 225-245. This is the non-parametric calculation for area under the ROC
curve, using the fact that a MannWhitney U statistic is closely related to
the area. In dismo, this is used in the gbm routines, but not elsewhere (see
evaluate).