Potts inference example

This example is to showcase some inference algorithms wrt their runtime and optimization performance.
I plan to have three examples:
* potts model - this one. a grid with attractive potentials
* tree - some random tree to exemplify the behavior for trees and chains
* complete - a comple graph with random potentials. this is much harder than the potts model an has more edges.
It could be that behavior is different here.

I didn't really try tweaking anything yet.
Also, we should probably reverse the sign of "energy". in the plot high is good, but usually low energy is good.