Extend dataframe groubpy aggregations to accept lists of column names

[stress-tess]

Extend dataframe groubpy aggregations to accept lists of column names. Leaving this empty will apply the aggregation to all columns

>>> username = ["Alice", "Bob", "Alice", "Carol", "Bob", "Alice"]
>>> userid = [111, 222, 111, 333, 222, 111]
>>> item = [0, 0, 1, 1, 2, 0]
>>> day = [5, 5, 6, 5, 6, 6]
>>> amount = [0.5, 0.6, 1.1, 1.2, 4.3, 0.6]
>>> p = pd.DataFrame({"userName": username, "userID": userid, "item": item, "day": day, "amount": amount})
>>> p.groupby('userID').first()
       userName  item  day  amount
userID
111       Alice     0    5     0.5
222         Bob     0    5     0.6
333       Carol     1    5     1.2