Unit testing for def/ module.

Added unit tests to test code in def/ module.

vendor/github.com/montanaflynn/stats/cumulative_sum.go

@@ -0,0 +1,21 @@
+package stats
+
+// CumulativeSum calculates the cumulative sum of the input slice
+func CumulativeSum(input Float64Data) ([]float64, error) {
+
+	if input.Len() == 0 {
+		return Float64Data{}, EmptyInput
+	}
+
+	cumSum := make([]float64, input.Len())
+
+	for i, val := range input {
+		if i == 0 {
+			cumSum[i] = val
+		} else {
+			cumSum[i] = cumSum[i-1] + val
+		}
+	}
+
+	return cumSum, nil
+}