Allow multiple traces per multiple users

[bbengfort]

Traces are generated with users and need to be associated with them. Allow an experiment generation technique that mixes and matches the trace with the number of user.

[bbengfort]

Note - so there was actually some hack code in the multi-sim library that allowed the passing of a directory of traces and it would assign traces to simulations based on finding the the number of users in the trace filename.

I think we're better off with the new mechanism:

1. specific traces in the simulation file
2. can be overrode by specifying a global trace file in multisim.