Fix isodata-thresholding according to Zachary Pincus

I'm not sure how fast this is, but you should save yourself a call and say:

```python
ts = threshold_isodata(imfloat, nbins=1024, return_all=True)
assert 0.49 < ts[0] < 0.51
assert all(0.49 < ts)
```

Since we've had to speed up our test suite before... =)