Fix some clippy warnings

src/model/difference.rs

@@ -65,7 +65,7 @@ impl DifferenceModel {
             panic!("observations must be added in chronological order");
         }
 
-        let var = var.unwrap_or_else(|| self.var);
+        let var = var.unwrap_or(self.var);
 
         let mut elems = self.process_items(winners, 1.0);
         elems.extend(self.process_items(losers, -1.0));