docs: 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)