Adding worker-based execution of simulations

[cchall]

When there are a very large number of workers running very short simulations it is much more efficient to have workers just execute the simulation directly rather than actually calling Executor.submit.

Technically, this workflow is possible right now if the user writes a python function that performs the setup and uses a subprocess. However, this requires creating a lot of setup that is normally handled by rsopt to generate new run files based on the work request.

A new execution_type could be created to allow the user to still call the code they would want to run (leveraging all the usual setup) but runing the simulation directly by the worker.