added definition and implementation for volume get value when referen…

…ced function

src/StockBench/indicators/volume/trigger.py

@@ -42,6 +42,9 @@ def add_to_data_from_rule_value(self, rule_value: str, side: str, data_manager:
         # volume does not require any additional data to be added to the data
         return
 
+    def get_value_when_referenced(self, rule_value: str, data_manager: DataManager, current_day_index,) -> float:
+        raise NotImplementedError('Volume cannot be referenced in a rule value')
+
     def check_trigger(self, rule_key, rule_value, data_manager, position, current_day_index) -> bool:
         """Trigger logic for volume.