docs: def get_reference_genome_file(refseqdir, build):
      """
      Convenience fxn to get reference genome from target dir, download if needed
      """
      if not os.path.exists(refseqdir) or not os.path.isdir(refseqdir):
          raise ValueError("No directory at {}".format(refseqdir))
      twobit_name = ''
      if build in ['b37', 'build 37', 'build37', '37', 'hg19']:
          twobit_name = 'hg19.2bit'
          build = 'build37'
      if not twobit_name:
          raise ValueError('Genome bulid "{}" not supported.'.format(build))
      twobit_path = os.path.join(refseqdir, twobit_name)
      if not os.path.exists(twobit_path):
          twobitdownload.save_genome('hg19', destdir=refseqdir)
      return twobit_path, twobit_name