se.py: Modify script to make multiprogramming much easier.

se.py: Modify script to make multiprogramming much easier.
Now, instead of --bench benchname, you can do --bench bench1-bench2-bench3 and it will
set up a simulation that instantiates those three workloads.  Only caveat is that now,
for sanity checking, your -n X must match the number of benches in the list.