fastmarching_0_implementing: Error in the Eikonal update

[glennib]

In https://github.com/gpeyre/numerical-tours/blob/master/matlab/fastmarching_0_implementing.ipynb on In [22], the Delta is calculated as
Delta = 2*W(j) - (dx-dy)^2;
while I believe it should be
Delta = 2*W(j)^2 - (dx-dy)^2;

This doesn't matter for the example with cell sizes of 1. However, in different cell sizes, it seems to cause a numerical approximation of the L^p norm close to the goal cell, where p equals the grid size.