Change approach global instead local

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

technical_tests/sand_grain_drop.py

@@ -39,9 +39,7 @@ def main(pile, n):
 
     while n != 0:
 
-        if pile[center][center] < 3:
-            pile[center][center] += 1
-
+        pile[center][center] += 1
         n -= 1
 
     return pile