generalize stack to support group keys

```python
>>> arr = ndtest('b=b0,b1')
>>> arr
b  b0  b1
    0   1
>>> stack({'a0..a1': arr, 'a2': 2, 'a3..a4': arr + 3}, 'a')
b\a   a0   a1   a2   a3   a4
 b0  0.0  0.0  2.0  3.0  3.0
 b1  1.0  1.0  2.0  4.0  4.0
```
see prototype in my "better_stack" (local) branch