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. 
Another nitpick by myself : The real version of the paper is used on 2D images as opposed to 3D volume like it is now made in the example and the new function. The mode/median part is just a hack I added myself for simplicuity and does not rely on any real theoretical background.

I try to specify that more explicitely in the functions, for example by removing the _ in the piesno_3D function.

Anyway, is it more of a convenience than anything else and seems to work so far on all our tests, but for spinal cord imaging for example, it is better to use the slice by slice version because of the huge gaps in the Z axis, which leads to a high variability in the standard deviation estimate of each slice.