small fixes

mhcflurry/antigen_presentation/__init__.py

-from .presentation_model import PresentationModel
+from .presentation_model import PresentationModel, build_presentation_models
 from .percent_rank_transform import PercentRankTransform
 from . import presentation_component_models, decoy_strategies
 
 __all__ = [
     "PresentationModel",
+    "build_presentation_models",
     "PercentRankTransform",
     "presentation_component_models",
     "decoy_strategies",

mhcflurry/antigen_presentation/presentation_component_models/expression.py

@@ -25,7 +25,7 @@ class Expression(PresentationComponentModel):
             group in expression_values.columns
             for group in experiment_to_expression_group.values())
 
-        assert_no_null(experiment_to_expression_group)
+        assert_no_null(expression_values)
 
         self.experiment_to_expression_group = experiment_to_expression_group
         self.expression_values = expression_values

mhcflurry/antigen_presentation/presentation_component_models/fixed_affinity_predictions.py

@@ -16,6 +16,9 @@ class FixedAffinityPredictions(PresentationComponentModel):
             - "value", "percentile_rank" (IC50 and percent rank)
             - peptide (string)
             - allele (string)
+
+    name : string
+        Used to name output columns and in debug messages
     """
 
     def __init__(