Distribution.from_shape is producing localarrays with size 0

With 4 engines:

``` python
In [1]: from distarray.dist import Context, Distribution

In [2]: context = Context()

In [3]: distribution = Distribution.from_shape(context, (9, 8), dist=('b', 'n'))

In [4]: distribution.localshapes()
Out[4]: [(3, 8), (3, 8), (3, 8), (0, 8)]
```

This appears to be from [this line](https://github.com/enthought/distarray/blob/master/distarray/metadata_utils.py#L143).

The size of this axis of the `grid_shape` is set to `comm_size`, which doesn't divide into the size of the corresponding axis of the DistArray.