Region extraction

Region extraction tools.  So far, implemented a mask-to-slice tool that will extract the minimal subcube given a validity mask.  It's really slow, though, because the mask array needs to be evaluated at every pixel... maybe that's necessary?  I feel like there should be some workaround.

For actual region extraction, though, one would specify xmin/xmax/ymin/ymax/zmin/zmax and therefore avoid having to go through the full cube mask evaluation.

cc @ChrisBeaumont, @astrofrog