fix cubic_hermite coefficients & powers

tests/integ.rs

@@ -131,6 +131,12 @@ fn linear_interpolation_several_keys() {
   assert_eq!(spline.clamped_sample(11.), Some(4.));
 }
 
+#[test]
+fn cubic_interpolation() {
+  let start = Key::new(0., 0., Interpolation::CatmullRom);
+  let end = Key::new(1., 10., Interpolation::default());
+}
+
 #[test]
 fn several_interpolations_several_keys() {
   let start = Key::new(0., 0., Interpolation::Step(0.5));