Unit testing for def/ module.

Added unit tests to test code in def/ module.

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

@@ -0,0 +1,18 @@
+package stats
+
+import "math"
+
+// Sigmoid returns the input values in the range of -1 to 1
+// along the sigmoid or s-shaped curve, commonly used in
+// machine learning while training neural networks as an
+// activation function.
+func Sigmoid(input Float64Data) ([]float64, error) {
+	if input.Len() == 0 {
+		return Float64Data{}, EmptyInput
+	}
+	s := make([]float64, len(input))
+	for i, v := range input {
+		s[i] = 1 / (1 + math.Exp(-v))
+	}
+	return s, nil
+}