Make API a bit prettier

tests/kickscore-basics.rs

@@ -5,7 +5,7 @@ use kickscore as ks;
 
 #[test]
 fn kickscore_basic() {
-    let mut model = ks::model::Binary::new(ks::model::binary::Observation::Probit);
+    let mut model = ks::model::Binary::probit();
 
     let k_spike = ks::kernel::Constant::new(0.5);
     let k_tom = ks::kernel::Exponential::new(1.0, 1.0);