Files outputted from voxel_vote aren't compressed

```sh
$ mincconvert -2 original_labels.mnc compressed_labels.mnc
$ ls -lh
-rw-r--r-- 1 gdevenyi gdevenyi 9.0M 2020-09-25 08:31 original_labels.mnc
-rw-rw-r-- 1 gdevenyi gdevenyi  45K 2020-09-25 10:52 compressed_labels.mnc
```

This may be pyminc's issue, I haven't checked yet.