[brookscorey] more efficient implementation of dkrn_dswe

dumux/material/fluidmatrixinteractions/2p/brookscorey.hh

@@ -269,8 +269,10 @@ public:
 
         swe = min(max(swe, 0.0), 1.0); // the equation below is only defined for 0.0 <= sw <= 1.0
 
-        return 2.0*(swe - 1)*(1 + (0.5 + 1/params.lambda())*pow(swe, 2.0/params.lambda())
-                                - (3.0/2.0 + 1/params.lambda())*pow(swe, 2.0/params.lambda()+1));
+        const auto lambdaInv = 1.0/params.lambda();
+        const auto swePow = pow(swe, 2*lambdaInv);
+        return 2*(swe - 1.0)*(1.0 + (0.5 + lambdaInv)*swePow - (1.5 + lambdaInv)*swePow*swe);
+
     }
 
 };