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. 
I just used the mode because most of the time you have something like 60%-75% of the slices with exactly the same sigma. The median could also be used I guess, I personaly sued it to double check that the mode worked correctly. 

I don't think they actually to use the same sigma value for the whole volume, it's just really less of an hassle than to consider each slice separately for any purpose one might have afterward.