Improvement to Meebit severing heads currently done at z=51

[elsewhat]

The heads are currently severed simply by taking everything above z=51

We want to improve this by

1. Creating an allow list of colors (such as light blue and blue hair).
2. An optional bounding box (needed for elephants)

Algorithm would be:
while(foundVoxelAtLevel)
for each voxel at zlevel in bounding box
if voxel.color is in allow list
cut from body and add to head
foundVoxelAtLevel= true
zlevel -= 1

Example of use case we want to improve