ENH/BUG groupby nth now filters, works with DataFrames
fixes #5552

partial for #5264

```
In [101]: df = pd.DataFrame([[1, np.nan], [1, 4], [5, 6]], columns=['A', 'B'])

In [102]: g = df.groupby('A')

In [103]: g.nth(0)
Out[103]:
   A   B
0  1 NaN
2  5   6

In [104]: g.nth(1)
Out[104]:
   A  B
1  1  4

In [105]: g.nth(-1)
Out[105]:
   A  B
1  1  4
2  5  6

In [106]: g.nth(0, dropna='any')  # old behaviour-like
Out[106]:
   B
A
1  4
5  6

In [107]: g.nth(1, dropna='any')  # old behaviour-like
Out[107]:
    B
A
1 NaN
5 NaN
```
