Strahler: Extend root detection to 3D

[tferr]

This is just an immediate followup to #4. Now that root-detection is working, it should be extended to 3D. In order to implement it, we will need to:

• [ ] Check if ROI is associated with all slices or just one (discussed here)
• [ ] Ignore end-points above/below the ROI, as needed
• [ ] Extend ImageProcessor operations to stack, as needed

[tferr]

This issue was moved to tferr/hIPNAT#2