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. 
Just out of curiosity (and the kind of data I have...): if this is not the case, for example if the data in the background gets masked out by the scanner, is it possible to use some of the brain data for this? For example, is it possible to use the ventricles?