Remove self loops in RAG

- Removes self loops in RAG
- Handles NCut case where self loops are required

This will result in much cleaner code, as seen in #1100
@jni 
For the call to `graph.rag_mean_color`
Your approach for the `coffee` image clocked `2.83`, `3.07`, `2.91` seconds
My approach clocked `2.27` , `2.16` and `2.19` seconds