Add method to check if item already has been added.

src/model.rs

@@ -47,6 +47,10 @@ impl BinaryModel {
         );
     }
 
+    pub fn contains_item(&self, name: &str) -> bool {
+        self.storage.contains_key(name)
+    }
+
     pub fn item_score(&self, name: &str, t: f64) -> (f64, f64) {
         let id = self.storage.get_id(name);
         let (ms, vs) = self.storage.item(id).fitter.predict(&[t]);