fix bugs in Gradient and test

spatial/r3/vector_test.go

@@ -306,8 +306,8 @@ func TestDivergence(t *testing.T) {
 
 func TestGradient(t *testing.T) {
 	const (
-		tol = 1e-12
-		h   = 1e-4
+		tol = 1e-6
+		h   = 1e-5
 	)
 	step := Vec{X: h, Y: h, Z: h}
 	rnd := rand.New(rand.NewSource(1))
@@ -316,7 +316,7 @@ func TestGradient(t *testing.T) {
 			p := randomVec(rnd)
 			got := Gradient(p, step, test.field)
 			want := test.gradient(p)
-			if vecApproxEqual(got, want, tol) {
+			if !vecApproxEqual(got, want, tol) {
 				t.Errorf("result out of tolerance. got %v, want %v", got, want)
 			}
 		}