Bone-region needs more precise segmentation

[jamesavery]

The coarse segmentation of the bone_region mask, which is then used to make the accurate SASS segmentation, is too imprecise: it segments part of the bone surface as resin due to the segmentation not being SASSed. So a preliminary SASS just for the bone is still in needed. Blood, resin, etc. is a big jumble before restricting to the bone_region mask, so only bone should be modeled at this point. However, simple thresholding is insufficient, even with heavy closing: outer bone is wrongly discarded.