Change default decision variables.

Change default decision variables to the more common set of {'Cost',
'Time'}. Users who need to include 'Carbon' and 'Energy' should
manually set the decision variables when defining their assessment
objects.

pelicun/model/loss_model.py

@@ -83,7 +83,7 @@ class LossModel(PelicunModel):
     def __init__(
         self,
         assessment: AssessmentBase,
-        decision_variables: tuple[str, ...] = ('Carbon', 'Cost', 'Energy', 'Time'),
+        decision_variables: tuple[str, ...] = ('Cost', 'Time'),
         dv_units: dict[str, str] | None = None,
     ) -> None:
         """