Add variable for electrify-sector total cost (#367)

Author: robertpietzcker

definitions/variable/economy/economy.yaml

@@ -146,6 +146,9 @@
 - Energy System Cost:
     description: Total energy system cost - including investments, O&M, and fuel costs
     unit: [billion USD_2010/yr, billion EUR_2020/yr]
+- Energy System Cost|Electricity:
+    description: Total energy system cost for electricity generation, including investment, O&M and fuel costs for power plants, grids, batteries
+    unit: [billion USD_2010/yr, billion EUR_2020/yr]
 
 - GDP|MER:
     description: GDP at market exchange rate