Handle overwriting results directory #261
Description
Sometimes users will encounter an error (e.g. #259 ) in their virtualship run simulations. They can then make their fixes and re-run the simulation. However, this will overwrite the results directory which already exists with the results up until the point of error, which can be inconvenient when the error occurs late in the expedition.
It would be good to provide an option not to overwrite the existing results, or to build on what already exists.