to speed up  "Saving notes" step

Although `notes` are not used, recommend
```python
import swifter
notes = notes.swifter.apply(process_frame_text, axis=1)
instead of
```
https://github.com/MLforHealth/MIMIC_Extract/blob/d8d2dea551283bea449b8495bfc1b5a41b90d837/mimic_direct_extract.py#L400
to speed up "Saving notes" step.
