support for multiple simulations in single output db

The EventManager generates a publicly accessible uuid for each simulation.  Backends are allowed to use/ignore it as fit.  I modified the Sqlite backend to generate an integer id associated with the uuid and every row in the database gets tagged with this simulation integer id.

By default now, cyclus will insert new simulation output inside an existing database along side previously existing simulation data rather than overwriting the sqlite db file with the new data.