Build ROC (receiver operating characteristic) curve from outer training
folds. Object can be plotted using plot() or passed to functions
pROC::auc() etc.
train_roc(x, direction = ""roc" object, see pROC::roc
Note: the argument outer_train_predict must be set to TRUE in
the original call to either nestcv.glmnet, nestcv.train or outercv.