Middle-equilibrium grid generator for U-Skate world

[riveSunder]

U-skate world has an equilibirum concentration of u and v that serves as the background for gliders like the eponymous u-glider.

where

$A = \frac{\sqrt{F}}{(F+k)}$

and the concentration of $u$ is

$u_{h3} = \frac{A - \sqrt{A^2 - 4}}{2A} $

and $v$

$v_{h3} = \frac{\sqrt{F} (A + \sqrt{A^2 - 4}) }{3}$

Munafo, R. (2014). Stable localized moving patterns in the 2-D Gray-Scott model. arXiv: Pattern Formation and Solitons.l. http://mrob.com/sci/papers/munafo2009-0410.pdf or https://arxiv.org/abs/1501.01990

[riveSunder]

Look to commit 51af7e9bc6510dae83f774e00eb4d949e9d084f4

[riveSunder]

Added in PR #74