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. 
That the PIESNO paper is mathematically quite involved is irrelevant to the tutorial. Except if you are going to explain why this is important. Anyway, I would remove that statement.