cec2020: implement SchafferExpanded func

bench/cec2020/basicFunctions.go

@@ -168,7 +168,17 @@ func Schaffer(x, y float64) float64 {
 }
 
 // SchafferExpanded is the "Expanded Schaffer's Function" of CEC2020.
-func SchafferExpanded(x []float64) float64 { return 0 }
+func SchafferExpanded(x []float64) float64 {
+	var sum float64
+
+	nx := len(x)
+
+	for i := range x {
+		sum += Schaffer(x[i], x[i%nx])
+	}
+
+	return sum
+}
 
 // Weierstrass is the "Weierstrass Function" of CEC2020 with a=0.5, b=3 and
 // kmax=20.