Grabcut Implementation

Grabcut (presented here) is a method of segmenting a reigion based on a sparse initial labeling.

A color model is employed (GMM is common) to determine color distibutions of the labeled reigions. These distributions are then used to asign proberbilities to unlabled regions of the image. Graphcut is then employed to optimize the boundries of the reigions based on the morphological boundries of the original image. This process may be iterated on until convergence.

This will be useful in MONAI for both interactive segmentation and as a post-processing step for segmentation algorithms.

I see this being split into three PRs. GMM, Graphcut, and finally Grabcut. Each would provide useful functionality induvidually.