Introduce output info class to support KPI flag

Created by: stefanoborini

Introduces the OutputSlotInfo to support the marking of KPIs without relying on KPICalculators.

force_bdss/core/output_slot_info.py

+from traits.api import HasStrictTraits, Bool
+
+from ..local_traits import Identifier
+
+
+class OutputSlotInfo(HasStrictTraits):
+    """
+    Class that specifies the name and characteristics of the output slots
+    of a data source.
+    This entity will go in the model object, and associates the positional
+    order in the containing list with the variable name that refers to the
+    value that should be taken.
+    """
+    #: The user defined name of the variable containing the value.
+    name = Identifier()
+
+    #: True if the value associated to this output slot must be exported as
+    #: a KPI.
+    kpi = Bool(False)