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. 
Probabilistic Identification and Estimation of Noise (PIESNO)