Introduced multilayer computation

Created by: stefanoborini

Introduces the possibility to perform more than one layer of DataSources

force_bdss/core/workflow.py

@@ -13,9 +13,12 @@ class Workflow(HasStrictTraits):
     #: Can be None if no MCO has been specified yet.
     mco = Instance(BaseMCOModel, allow_none=True)
 
-    #: Contains the factory-specific DataSource Model objects.
-    #: The list can be empty
-    data_sources = List(BaseDataSourceModel)
+    #: The execution layers. Execution starts from the first layer,
+    #: where all data sources are executed in sequence. It then passes all
+    #: the computed data to the second layer, then the third etc.
+    #: For now, the final execution is performed by the KPI layer, but this
+    #: will go away when we remove the KPI calculators.
+    execution_layers = List(List(BaseDataSourceModel))
 
     #: Contains the factory-specific KPI Calculator Model objects.
     #: The list can be empty