Produce joint called variants from input gVCF files.

def run_gvcftyper(vrn_files, out_file, region, data):
    """Produce joint called variants from input gVCF files.
    """
    if not utils.file_exists(out_file):
        with file_transaction(data, out_file) as tx_out_file:
            license = license_export(data)
            ref_file = dd.get_ref_file(data)
            input_files = " ".join(vrn_files)
            region = bamprep.region_to_gatk(region)
            cmd = ("{license}sentieon driver -r {ref_file} --interval {region} "
                   "--algo GVCFtyper {tx_out_file} {input_files}")
            do.run(cmd.format(**locals()), "Sentieon GVCFtyper")
    return out_file