Some of the "built-in" methods for dask SeriesGroupBy objects are missing.

In pandas we could do something like...

import numpy as np
import pandas as pd

df = pd.DataFrame({
    'a_cat': list('aabbbaccc'),
    'b_num': [np.nan if i%3!=0 else (i+1) for i in range(9)]
})

df.groupby('a_cat')['b_num'].ffill()

But when I try to do it in Dask..

import dask.dataframe as dd

ddf = dd.from_pandas(df)
ddf.groupby('a_cat')['b_num'].ffill()

I receive an AttributeError: 'SeriesGroupBy' object has no attribute 'ffill'

I'm hoping this can be implemented along with the other fillna methods. Thanks.