Asyncio Improvements

Possibly should do this w.r.t. new SubprocessManager

```
converter.py
21 def convert_sync(self, in_resource, out_resource):
22 # TODO: Make both optional (run_until_complete here)
23 msg = 'Converter subclass must override at least convert_sync.'
24 raise NotImplementedError(msg)
```

- [ ] AsyncIO improvements
- [ ] Replace all file system calls with aiofiles
- [ ] Replace all subprocess.run system calls with asyncio equivalent

Start with converter.py around convert_sync and review the referenced new SubprocessManager. Locate the filesystem calls and subprocess.run calls covered by the checklist, then determine the async replacements needed. Done means the listed filesystem and subprocess operations use asyncio-compatible approaches and the checklist is complete.

Written by the indexing model from the issue text.