`BlockIndex`: support negative indices in `iter_select` selections


```python
In : bi
<arraykit.BlockIndex(blocks: 5, rows: 4, columns: 5, dtype: dtype('O'))>
In : bi.to_list()
[(0, 0), (1, 0), (2, 0), (3, 0), (4, 0)]
In : tuple(bi.iter_select([0, 1]))
((0, 0), (1, 0))
In : tuple(bi.iter_select([0, -1]))
IndexError: index out of range
```
