Created by Julien Palard

Simulate gravity applied to a sandpile.

Write a function `apply_gravity`

that given a 2D numpy array representing
sand will "apply gravity" on it (and returns nothing):

- Each element of the array is an integer representing the height of the sandpile
- Any "pile" that has 4 or more sand particles on it collapses, resulting in four particles being subtracted from the pile and distributed among its neighbors.

```
>>> import numpy as np
>>> sandpile = np.zeros((5, 5), dtype=np.uint32)
>>> sandpile[2, 2] = 16
>>> apply_gravity(sandpile)
>>> print(sandpile)
[[0 0 1 0 0]
[0 2 1 2 0]
[1 1 0 1 1]
[0 2 1 2 0]
[0 0 1 0 0]]
```

https://en.wikipedia.org/wiki/Abelian_sandpile_model

Use matplotlib to display the result as an image:

```
apply_gravity(sandpile)
plt.imshow(sandpile)
plt.show()
```

To showcase the results, computed a pile of 2 ** 24 grains of sand, beware it took 14 days on my laptop…

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