Add new phantoms

Hi, I stated generating simple (some more complicated) phantoms. I think it can be interesting to share those phantoms. At some point, I would also like to share DWIs at various SNR + processed data + masks for all phantoms.

I putted 20 phantoms in the examples folder. No need to rush on this PR, I just wanted to share what I have done so far.

Have a look to the phantoms here: https://docs.google.com/document/d/1pOmPn3xFvpemcDp5EeS6fs3jVf7dSXqj72VWmqEkM-w/edit?usp=sharing
(phantomas_dwis [...] --res 2 --snr 0 --fov 110)

I commited all models that I have done, maybe we could focus on, say 10 'best' phantoms.

Tell me what you think. Which configurations are missing? Should it be in the examples folder, or somewhere else?