New PIESNO example and small corrections

The PIESNO method was merged in Dipy without an example. Here is an example for the Dipy Gallery. 

At the same time, the PIESNO method was corrected by @samuelstjean to better match the Koay et al 2009 paper. Other examples have also been uniformized. 
Not personally for the public released datasets (I'm playing with a spinal cord one currently). Do they say the hardware specs of their scanner somewhere? They have a ton of papers about that, for example http://onlinelibrary.wiley.com/doi/10.1002/mrm.24623/abstract;jsessionid=1A58CFCAB23677366853EE1BC46F011A.d03t02

They used a SENSE based one on a 32 channel coils, so if they always used that one, and this is the right human connectome project (there are two of them), that would mean N=1 because of SENSE.

Another way to check that is just draw a manual roi, then check the nosie profile. It's easy to see if it's rician or not by checking the curve. In the latter case, it's harder to guess the value of N (anyway I can't).