add comment for change in parmest

Added comment to be explicit that dict.fromkeys() is used instead of set() to preserve parameter ordering and thus make the problem deterministic.

Author: djlaky

pyomo/contrib/parmest/parmest.py

@@ -309,6 +309,7 @@ def __init__(
         for experiment in self.exp_list:
             model = experiment.get_labeled_model()
             theta_names.extend([k.name for k, v in model.unknown_parameters.items()])
+        # Utilize list(dict.fromkeys(theta_names)) to preserve parameter order compared with list(set(theta_names)), which had nondeterministic ordering of parameters
         self.estimator_theta_names = list(dict.fromkeys(theta_names))
 
         self._second_stage_cost_exp = "SecondStageCost"