Rename KeyPerformanceCalculator to DataSource

Created by: stefanoborini

Renamed Key Performance Calculator to DataSource, as this is the name we agreed upon in EnginSoft meeting.

examples/test_workflow.json

@@ -2,7 +2,7 @@
     "multi_criteria_optimization": {
         "name": "basic"
     },
-    "key_performance_indicators": [
+    "data_sources": [
         "viscosity", "price"
     ]
 }