Ensure CRAM file has a .crai index file.

def index(in_cram, config):
    """Ensure CRAM file has a .crai index file.
    """
    out_file = in_cram + ".crai"
    if not utils.file_uptodate(out_file, in_cram):
        with file_transaction(config, in_cram + ".crai") as tx_out_file:
            tx_in_file = os.path.splitext(tx_out_file)[0]
            utils.symlink_plus(in_cram, tx_in_file)
            cmd = "samtools index {tx_in_file}"
            do.run(cmd.format(**locals()), "Index CRAM file")
    return out_file