Spawning hexagonal cells in the simulation

[diyasrivas]

change draw_cell function to spawn hexagons

[edoardogiacomello]

Hi @diyasrivas, a couple of things before we can merge:

1. Can you unstage your changes to the examples folder (except for the example.ipynb)? It is nice to have some samples but I wouldn't push them to the repository right now, otherwise the tree would become heavier...
2. smallest_square_crop was meant for cropping individual cells to run the RD in it, I don't think hexagons should be involved in that function, unless you plan to repurpose it to make something else.

Feel free to get back to me if you need clarifications...

Thank you!