Groupby object does not allow subscripting unlike pandas

**Description**
Encountered an error when trying to access a column in an eland DataFrame using the `groupby()` method.

`grouped_data = df.groupby('feature')['used']`

**Expected object**
`pandas.core.frame.DataFrame`

**Actual behaviour**
Throws TypeError
`---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
~\AppData\Local\Temp\ipykernel_20028\2953508713.py in
1 #feature_max = df.groupby('feature')['used'].agg(['min', 'max'])
2 #print(feature_max)
----> 3 grouped_data = df.groupby('feature')['used'].agg(max_value='max')

TypeError: 'DataFrameGroupBy' object is not subscriptable`

Start at the groupby() entry point and the DataFrameGroupBy object shown in the traceback, then reproduce df.groupby('feature')['used'].agg(max_value='max'). Done means column subscripting works like the reported pandas usage and the aggregation no longer raises TypeError.

Written by the indexing model from the issue text.