Simulate a 2d [Ising Ferromagnet](https://en.wikipedia.org/wiki/Ising_model).

Use <UP>  / <DOWN> to change the temperature.
The Curie temperature is at *T = 1.0*.

Implements the [Metropolis-Hastings algorithm](https://en.wikipedia.org/wiki/Metropolis%E2%80%93Hastings_algorithm).