Use array impl instead of vec

tests/nba-history.rs

@@ -61,7 +61,7 @@ fn nba_history() {
     let mut model = ks::model::Binary::new(ks::model::binary::Observation::Probit);
 
     for team in teams {
-        let kernel: Vec<Box<dyn ks::Kernel>> = vec![
+        let kernel: [Box<dyn ks::Kernel>; 2] = [
             Box::new(ks::kernel::Constant::new(0.03)),
             Box::new(ks::kernel::Matern32::new(0.138, 1.753 * seconds_in_year)),
         ];