Propation is right for small iteration and/or pile with low grain rate.
For pile with high grain rate, the main while loop is not enough to make propagation
in all cells
Structure for pile walking to find cells greater than 3 is set
I do not update neighbors cells, then only central value of the pile (where the
sand grain is dropped) is at the right value in the end
1st idea was to manage propagation cells after cells. But this looks to generate
a chain reaction that I cannot manage in a function.
New idea is to manage the whole pile :
* add a grain
* find cells that reached 4
* change cell & update neighbors
* find cells that reached 4
* change cell & update neighbors
Freezed