VariableMap: Use refs for args again

Author: alexdewar

src/simulation/optimisation.rs

@@ -35,8 +35,8 @@ impl VariableMap {
     /// Get the [`Variable`] corresponding to the given parameters.
     // **TODO:** Remove line below when we're using this
     #[allow(dead_code)]
-    fn get(&self, asset: AssetRef, time_slice: TimeSliceID) -> Variable {
-        let key = (asset, time_slice);
+    fn get(&self, asset: &AssetRef, time_slice: &TimeSliceID) -> Variable {
+        let key = (asset.clone(), time_slice.clone());
 
         *self
             .0