Make database for gffutils. 

    Parameters
    ----------
    gtf : str
        Path to Gencode gtf file.

    db : str
        Path to save database to. 

    Returns
    -------
    out_db : gffutils.FeatureDB
        gffutils feature database.

def make_gffutils_db(gtf, db):
    """
    Make database for gffutils. 

    Parameters
    ----------
    gtf : str
        Path to Gencode gtf file.

    db : str
        Path to save database to. 

    Returns
    -------
    out_db : gffutils.FeatureDB
        gffutils feature database.

    """
    import gffutils
    out_db = gffutils.create_db(gtf,
                                db,
                                keep_order=True,
                                infer_gene_extent=False)
    return out_db