Is it right there are no tests for these functions:

```
quantize_positive_image
quantize_positive_volume
compute_masked_image_class_stats
compute_masked_volume_class_stats
```

I know that it's often tiring to test these low-level functions, but the problem is, if they aren't tested, then it becomes very hard to maintain the code, because other people coming to the code won't know if they have broken it, so will tend to avoid it, and won't be able to improve, fix it as easily.