fix weighted covariance implementation

Weighted covariance accidentally used squared weights.  Added a test
case and fixed implementation.

stat_test.go

@@ -128,6 +128,12 @@ func TestCovariance(t *testing.T) {
 			weights: []float64{1, 1.5, 1},
 			ans:     3.2,
 		},
+		{
+			p:       []float64{1, 4, 9},
+			q:       []float64{1, 4, 9},
+			weights: []float64{1, 1.5, 1},
+			ans:     13.142857142857146,
+		},
 	} {
 		c := Covariance(test.p, test.q, test.weights)
 		if math.Abs(c-test.ans) > 1e-14 {