Option to never overwrite an existing file

[jburos]

Sometimes you want to make sure an existing model run will never overwrite a previous file.

Right now, users can label specific model runs using the model_name parameter, which will change the prefix of all runs. And right now if a cache file exists it will not get overwritten because the result will instead read from that file. But perhaps an overwrite parameter would be helpful; this would force re-execution if TRUE, and never overwrite an existing file (but read from that file) if FALSE.