cec2020: implement Griewank func

bench/cec2020/basicFunctions.go

@@ -19,7 +19,19 @@ func HGBat(x []float64) float64 { return 0 }
 func Rosenbrock(x []float64) float64 { return 0 }
 
 // Griewank is the "Griewank's Function" of CEC2020.
-func Griewank(x []float64) float64 { return 0 }
+// ref: https://www.sfu.ca/~ssurjano/griewank.html
+func Griewank(x []float64) float64 {
+	var sum float64
+
+	var prod float64
+
+	for i := range x {
+		sum += math.Pow(x[i], 2) / 4000
+		prod -= math.Cos(x[i] / math.Sqrt(float64(i)))
+	}
+
+	return sum - prod + 1
+}
 
 // Ackley is the "Ackley's Function" of CEC2020.
 func Ackley(x []float64) float64 { return 0 }