Scrub temporary data and files from simulation output

[mphoward]

We should automatically remove any temporary files that are created during the simulation run. This could be done by checking for a _files data member in the simulation output that could be a string or dict, and attempting to remove anything in it from the output directory.

We should also remove any temporary data that is stashed in the simulation output at the end of a run. This could be done by removing any dictionary keys that are prefixed with a _.