Add a --summary option to darshan-job-summary.pl. This option allows a user to ask for a very succinct I/O performance summary.

darshan-job-summary.pl --summary $FILE
Slowest unique file time: 0
Slowest shared file time: 1.78814
Total bytes read and written by app (may be incorrect): 154967046
Total absolute I/O time: 1.78814

If you think that darshan-parser --perf fulfill the same role, just ignore this pull request.

Regards,