Introduced ExecutionLayer object to simplify notification handling

Created by: stefanoborini

Introduces the ExecutionLayer object in the model to simplify handling at the UI level

force_bdss/core/execution_layer.py

+from traits.api import HasStrictTraits, List
+
+from force_bdss.data_sources.base_data_source_model import BaseDataSourceModel
+
+
+class ExecutionLayer(HasStrictTraits):
+    """Represents a single layer in the execution stack.
+    It contains a list of the data source models that must be executed.
+    """
+    data_source_models = List(BaseDataSourceModel)