Add the capability of changing the initial working directory of `SubprocessTao`

It would be nice to be able to launch `SubprocessTao` starting in a given directory.

For example:

```python
SubprocessTao(lattice_file="lat.bmad", cwd="/path/to/lattice")
```

Where `/path/to/lattice/lat.bmad` exists, and all other file references would be relative to `/path/to/lattice`.

I think calling it `cwd` might be sensible with the parallels to how subprocesses are typically launched in Python. Alternatively, `workdir` or `init_dir` or something could be used.

Start at the SubprocessTao constructor and follow the process-launch entry point to see how its arguments and working directory are handled. Done means SubprocessTao accepts the proposed directory argument, launches from that directory, and resolves the lattice file and other file references relative to it.

Written by the indexing model from the issue text.