add_process doesn't like being passed a path

`add_process` can accept a path to a file to run, but if you try to pass in a PosixPath object instead of a string will result in this error:

```
  File "/home/dev/.pyenv/versions/3.10.13/lib/python3.10/site-packages/simvue/run.py", line 490, in add_process
    _cmd_list += [f"-{kwarg}{(' '+val) if val else ''}"]
```

Could this be fixed to also accept path objects?
