Download GTF file from Ensembl, only keeping the chr17 entries.

def get_gtf():
    """
    Download GTF file from Ensembl, only keeping the chr17 entries.
    """
    size, md5, url = GTF
    full_gtf = os.path.join(args.data_dir, os.path.basename(url))
    subset_gtf = os.path.join(
        args.data_dir,
        os.path.basename(url).replace('.gtf.gz', '_%s.gtf' % CHROM))

    if not _up_to_date(md5, subset_gtf):
        download(url, full_gtf)
        cmds = [
            'zcat', '<',
            full_gtf,
            '|', 'awk -F "\\t" \'{if ($1 == "%s") print $0}\''
            % CHROM.replace('chr', ''),
            '|', 'awk \'{print "chr"$0}\'', '>', subset_gtf]

        logged_command(cmds)