fix error in ROCSS computation

include/evalhyd/detail/probabilist/contingency.hpp

@@ -514,7 +514,9 @@ namespace evalhyd
             {
                 // compute the area under the ROC curve
                 // xt::trapz(y, x, axis=4)
-                auto A = xt::trapz(POD, POFD, 4);
+                // (note: taking the opposite of the integration results
+                //        because POD/POFD values are in decreasing order)
+                auto A = - xt::trapz(POD, POFD, 4);
 
                 // compute the ROC skill score
                 // $SS_{ROC} = \frac{A - A_{random}}{A_{perfect} - A_{random}}$