parallel: MacOS multiprocessing doesn't work but multiprocess might

From the notes, I'm just copying and haven't tested:

The error seen was:
```
Process SpawnPoolWorker-1:
Process SpawnPoolWorker-2:
Process SpawnPoolWorker-3:
Traceback (most recent call last):
```

Someone commented:
```
    - Yes it seems that with Mac multiprocessing is not working and you need to import multiprocess (a fork of multiprocessing).
```