Added tests to constant kernel

src/kernel/exponential.rs

@@ -14,6 +14,10 @@ impl Exponential {
 }
 
 impl Kernel for Exponential {
+    fn k_mat(&self, _ts1: &[f64], _ts2: Option<&[f64]>) -> ArrayD<f64> {
+        unimplemented!();
+    }
+
     fn k_diag(&self, ts: &[f64]) -> Array1<f64> {
         Array1::ones(ts.len()) * self.var
     }