Skip nesting coefficient evaluation when evaluating logsums using MNL models (#909)

* Update logsums.py to only evaluate nesting coefficients if the logsums are being calculated with a nested logit model

* Black abm\models\util\logsums.py

Author: JoeJimFlood

activitysim/abm/models/util/logsums.py

@@ -167,7 +167,10 @@ def compute_location_choice_logsums(
     )
 
     nest_spec = config.get_logit_model_settings(logsum_settings)
-    nest_spec = simulate.eval_nest_coefficients(nest_spec, coefficients, trace_label)
+    if nest_spec is not None:  # nest_spec is None for MNL
+        nest_spec = simulate.eval_nest_coefficients(
+            nest_spec, coefficients, trace_label
+        )
 
     locals_dict = {}
     # model_constants can appear in expressions