code: def get_dexseq_gff(config, default=None):
      
      dexseq_gff = tz.get_in(tz.get_in(['dexseq_gff', 'keys'], LOOKUPS, {}),
                             config, None)
      if not dexseq_gff:
          return None
      gtf_file = get_gtf_file(config)
      if gtf_file:
          base_dir = os.path.dirname(gtf_file)
      else:
          base_dir = os.path.dirname(dexseq_gff)
      base, _ = os.path.splitext(dexseq_gff)
      gff_file = os.path.join(base_dir, base + ".gff")
      if file_exists(gff_file):
          return gff_file
      gtf_file = os.path.join(base_dir, base + ".gff3")
      if file_exists(gtf_file):
          return gtf_file
      else:
          return None
docs: some older versions of the genomes have the DEXseq gff file as
      gff instead of gff3, so this handles that by looking for either one