SA: Tag Junction voxels and use ParticleAnalyzer (PA) to count branches
Advantages:
 - Bitmap image of junctions
 - Consistency: All segments flanked by a junction are counted
   (previously, we're retrieving a mixed-bag of disconnected and inter-
   connected segments)
Disadvantages:
 - While analysis is performed in 3D, counts are performed in a 2D MIP
 - Because junction voxels are eliminated from image (so that PA can
   perform the count), small branches (2-4 pixels) can be ignored