Fix results aggregation, supply-demand interaction

Aggregating according to a simplified list of heating technologies must
sum up any grouped technologies.

Fix is to test if entry exists in dict and create if missing,
otherwise add values rather than overwriting.

energy_demand/basic/demand_supply_interaction.py

@@ -64,13 +64,17 @@ def constrained_results(
     # ----------------------------------------
     for submodel_nr, submodel in enumerate(submodels_names):
         for tech, fuel_tech in results_constrained.items():
-            # Technological simplifications because of different technology definition and because not all technologies are used in supply model
+            # Technological simplifications because of different technology
+            # definition and because not all technologies are used in supply model
             tech_simplified = model_tech_simplification(tech)
             fueltype_str = technologies[tech_simplified].fueltype_str
             fueltype_int = technologies[tech_simplified].fueltype_int
             key_name = "{}_{}_{}".format(submodel, fueltype_str, tech_simplified)
 
-            supply_results[key_name] = fuel_tech[submodel_nr][:, fueltype_int, :]
+            if key_name not in supply_results:
+                supply_results[key_name] = fuel_tech[submodel_nr][:, fueltype_int, :]
+            else:
+                supply_results[key_name] += fuel_tech[submodel_nr][:, fueltype_int, :]
 
     assert not testing_functions.test_if_minus_value_in_array(results_unconstrained_no_heating)