Make functions to make working with multiple scales easier

[jamesavery]

We want to calculate geometric info, e.g. implant-FoR.py, cylinder_surface.py, etc. in coarse resolution. However, their coarse results should be used to classify full-resolution voxels. Needs simple functions to match

• masks of different scales
• nonzeros-index lists of different scales

Also, we may want to do

• anti-aliasing low-resolution masks