Numerical methods to partial differential equations

	A center point (i,j) has 4 nearest neighbors. For a uniform system If it is none-uniform. In a periodic boundary conditions (PBCs), the top and the bottom are connected, so do the left and the right. All elements are treated as inners. When a zero-flux border Element(i,j) is visited, do the same thing, with an additional border visit to do this: Each corner is treated twice as a border element, no additional work needed.