Parallel processing in groupby operations
Hello,

It would be nice to have the possibility to do parallel processing using `joblib` when applying something to a groupby. I'm dreaming about a `parallel` optional argument which defaults to `False`.

I think it's best to have that parallel argument while grouping

The only option I have now is to make list on the groupby results, apply a function on that list using joblib, and concatenate the results. It leads to unefficient and difficult to read code.

```python

############
# Examples #
############
data.groupby(...).fillna(..., parallel=True)
data.groupby(...).apply(..., parallel=True)
data.groupby(...).whatever(..., parallel=True)
data.groupby(...).whatever(...) # not parallel
```

