Region Adjacency Graph

## Example Images
![figure_1](https://cloud.githubusercontent.com/assets/3823490/3291718/5ce9b952-f587-11e3-8c29-e2aace6d7794.png)
![figure_2](https://cloud.githubusercontent.com/assets/3823490/3291723/7074b134-f587-11e3-8c73-d28222891a21.png)


Yes. What I meant is that the typical use case would be to build a RAG and then threshold it, so no merges take place. However, I can see a use case where you do some initial merging according to some criterion, and *then* threshold the graph, so this could be useful. However, you still could do the `map_array` approach above.