QST: Such a behavior of DataFrameGroupBy.cumcount() is intended ??
#### Question about pandas

Let's say we have a `DataFrame` named by `pdf` like the below.

```python
>>> pdf = pd.DataFrame([['a'], ['a'], ['a'], ['b'], ['b'], ['a']], columns=list('A'))
>>> pdf
   A
0  a
1  a
2  a
3  b
4  b
5  a
```

For above `pdf`, `GroupBy.cum~` functions seems not work, but raise `DataError`

```python
# GroupBy.cummax
>>> pdf.groupby("A").cummax()
Traceback (most recent call last):
...
pandas.core.base.DataError: No numeric types to aggregate

# GroupBy.cummin
>>> pdf.groupby("A").cummin()
Traceback (most recent call last):
...
pandas.core.base.DataError: No numeric types to aggregate

# GroupBy.cumprod
>>> pdf.groupby("A").cumprod() 
Traceback (most recent call last):
...
pandas.core.base.DataError: No numeric types to aggregate

# GroupBy.cumsum
>>> pdf.groupby("A").cumsum()
Traceback (most recent call last):
...
pandas.core.base.DataError: No numeric types to aggregate
```

And I thought that `GroupBy.cumcount()` also wouldn't work, but It's actually working like the below.

```python
>>> pdf.groupby("A").cumcount()
0    0
1    1
2    2
3    0
4    1
5    3
dtype: int64
```

Would someone let me know if Is it intended or unexpected behavior ?

Thanks.