Remove need to box kernel on model

examples/nba-history.rs

@@ -62,7 +62,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
             Box::new(ks::kernel::Matern32::new(0.138, 1.753 * seconds_in_year)),
         ];
 
-        model.add_item(&team, Box::new(kernel));
+        model.add_item(&team, kernel);
     }
 
     for (winner, loser, t) in observations {