Flow tracker now only produces one LinkUtilizationEvent per link, instead of one per flow.
Script should now kill running processes more reliably.

Script now longer terminates if it tries to kill an invalid process ID
(often happened when vlc-wrapper was killed, and the script tried to
kill the wrapped vlc process next)
Modified logging to make it easier to debug flow replacement.
Sims which fail due to network connectivity are now automatically rerun.