Given a tuple list [(pathway, feature)], create a dictionary
        mapping each feature to the pathways overrepresented in the feature.

def _collect_pathways_by_feature(self, pathway_feature_tuples):
        """Given a tuple list [(pathway, feature)], create a dictionary
        mapping each feature to the pathways overrepresented in the feature.
        """
        pathways_in_feature = {}
        for (pathway, feature) in pathway_feature_tuples:
            vertex = self.add_pathway(pathway)
            if feature not in pathways_in_feature:
                pathways_in_feature[feature] = []
            pathways_in_feature[feature].append(vertex)
        return pathways_in_feature