code: def prepare_rsem_reference(gtf, multifasta, build):
      
      if not utils.which("rsem-prepare-reference"):
          logger.info("Skipping prepping RSEM reference because "
                      "rsem-prepare-reference could not be found.")
          return None
  
      command = PREPARE_REFERENCE.format(gtf=gtf, multifasta=multifasta,
                                         build=build)
      with transaction.tx_tmpdir(remove=False) as rsem_genome_dir:
          with utils.chdir(rsem_genome_dir):
              message = "Preparing rsem reference from %s" % gtf
              do.run(command, message)
      return rsem_genome_dir
docs: gtf: path to GTF file (must have gene_id and transcript_id)
      multifasta: path to multifasta file
      build: name of organism build (e.g. hg19)