Read in the significant pathways file as a
    pandas.DataFrame.

def _load_significant_pathways_file(path_to_file):
    """Read in the significant pathways file as a
    pandas.DataFrame.
    """
    feature_pathway_df = pd.read_table(
        path_to_file, header=0,
        usecols=["feature", "side", "pathway"])
    feature_pathway_df = feature_pathway_df.sort_values(
        by=["feature", "side"])
    return feature_pathway_df