Simulator should accept a PfsConfig instance

The Simulator has the capability of reading a library of spectra, but the association between fibers and spectra is currently random. In order to simulate a survey, the user should provide a PfsConfig to the Simulator, which it then implements.
This requires:

• Modifying the front-end of the Simulator to accept a PfsConfig
• Providing a facility to construct a PfsConfig (random assignment) for the user who doesn't want to run a full simulated survey targeting program.