Implement more and more logic.

src/kernel/constant.rs

@@ -1,3 +1,5 @@
+use ndarray::prelude::*;
+
 use super::Kernel;
 
 pub struct Constant {
@@ -10,4 +12,28 @@ impl Constant {
     }
 }
 
-impl Kernel for Constant {}
+impl Kernel for Constant {
+    fn k_diag(&self, ts: &[f64]) -> Array1<f64> {
+        Array1::ones(ts.len()) * self.var
+    }
+
+    fn order(&self) -> usize {
+        1
+    }
+
+    fn state_mean(&self, t: f64) -> Array1<f64> {
+        Array1::zeros(1)
+    }
+
+    fn state_cov(&self, t: f64) -> Array2<f64> {
+        array![[1.0]] * self.var
+    }
+
+    fn measurement_vector(&self) -> Array1<f64> {
+        array![1.0]
+    }
+
+    fn transition(&self, t0: f64, t1: f64) -> Array2<f64> {
+        todo!();
+    }
+}