use a GTF file and a reference FASTA file to index the transcriptome

def index_transcriptome(gtf_file, ref_file, data):
    """
    use a GTF file and a reference FASTA file to index the transcriptome
    """
    gtf_fasta = gtf.gtf_to_fasta(gtf_file, ref_file)
    bowtie2_index = os.path.splitext(gtf_fasta)[0]
    bowtie2_build = config_utils.get_program("bowtie2", data["config"]) + "-build"
    cmd = "{bowtie2_build} --offrate 1 {gtf_fasta} {bowtie2_index}".format(**locals())
    message = "Creating transcriptome index of %s with bowtie2." % (gtf_fasta)
    do.run(cmd, message)
    return bowtie2_index