Snr example simplifications

This PR simplifies the snr_in_cc.py example. I believe it is much clearer and shows how one can estimate a worst-case/best-case SNR from DWI data.

I have removed a lot of text and unecessary lines. Also, the segmentation part of the CC is not robust and is clearly not reproducible easily on other dataset. I have removed this part from the original example. Now, it is just an example to compute SNR. 

I still find this example long. Maybe people can help reducing the number of lines of code when defining ROIs.