Sort ROIs by proximity

[jakirkham]

Sometimes we are running into a situation where neighboring ROIs are the same color. My suspicion is this occurs because ROIs that are neighboring spatially may not have neighboring indices. The coloring algorithm tries to color neighboring indices with starkly different colors. However, if two ROIs are far a part in terms of indices, they may end up with more similar colors. Hence trying to clump spatial close ROIs in the array should counteract this effect.