Refactor assignment of Helium population updated values

tardis/plasma/properties/level_population.py

@@ -98,5 +98,5 @@ def calculate(
             partition_function,
         )
         if helium_population_updated is not None:
-            level_number_density.loc[2].update(helium_population_updated)
+            level_number_density.loc[2, helium_population_updated.columns] = helium_population_updated.values
         return level_number_density