def get_gtf_db(gtf, in_memory=False):
    """
    create a gffutils DB
    """
    db_file = gtf + '.db'
    if gtf.endswith('.gz'):
        db_file = gtf[:-3] + '.db'
    if file_exists(db_file):
        return gffutils.FeatureDB(db_file)
    db_file = ':memory:' if in_memory else db_file
    if in_memory or not file_exists(db_file):
        debug('GTF database does not exist, creating...')
        infer_extent = guess_infer_extent(gtf)
        db = gffutils.create_db(gtf, dbfn=db_file,
                                infer_gene_extent=infer_extent)
        return db
    else:
        return gffutils.FeatureDB(db_file)
create a gffutils DB