tensor voting now works with masked images

Tensor voting (to detect thin surfaces, like membranes) now works when the -mask argument is used. (This feature was supposed to work, but I had never tested it until today.) The -mask argument us used to limit the detection of objects to certain volumes within the image.

In practice, those volumes are often chosen to be membrane-bound compartments which were detected by previous iterations of running filter_mrc (and voxelize_mesh.py). Consequently this feature makes it much easier to segment images containing multiple concentric objects inside each other. One can now use these tools to segment Russian-doll-like membrane bound compartments inside of cells or other organelles.

(I'm currently working on a short tutorial to demonstrate these features on a small tomogram. If I get that ready, I will post it elsewhere. Probably it will be a separate github project.)