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. 
One again, the slice by slice in a for loop is mostly for the end user convenience, and not backed in any way by theory. One can call the _piesno_3D function himself to get a slcie by slice estimate (as desired in spinal cord dMRI), so I'll think a a way to write that as a note in the docstring.