Peak directions update

I 've been playing lately a lot with peak_directions and found out that our method was not as robust as it could be. The main idea is that peak directions gives better results when it is applied on a min_max normalized odf. With this in mind I have updated the peak_directions function and tested it thoroughly. 

In addition, I am introducing a new function for peak visualization that can work together with sphere_funcs from fvtk (with tests and example of CSD peaks). 

Let me know what you think.