Adding Mean and MeanBy (#414)

* feat: create Mean for ints and floats

* feat: create MeanBy for ints and floats

* chore: add example tests for mean Mean and MeanBy

math_example_test.go

@@ -66,3 +66,21 @@ func ExampleSumBy() {
 	fmt.Printf("%v", result)
 	// Output: 6
 }
+
+func ExampleMean() {
+	list := []int{1, 2, 3, 4, 5}
+
+	result := Mean(list)
+
+	fmt.Printf("%v", result)
+}
+
+func ExampleMeanBy() {
+	list := []string{"foo", "bar"}
+
+	result := MeanBy(list, func(item string) int {
+		return len(item)
+	})
+
+	fmt.Printf("%v", result)
+}