Analyse pixel value distribution within different regions/structures

[Mrinal Jain]

For example: how are the pixels inside a "Mandible" are distributed?

This will be at two levels:

1. For individual patients, how does the pixel (or "voxel") value vary across different regions. Added in commit e0505dc
2. Across all patients, what's the distribution of these pixels for each region. *Added in commit 7dca68e

This could also be useful for creating very simple baseline models, essentially based on stuff like thresholding. Will also be helpful for windowing.