LiFE

This is just to let you know that I am working on an implementation of the Linear Fascicle Evaluation algorithm, described in our recent paper: http://www.nature.com/nmeth/journal/vaop/ncurrent/full/nmeth.3098.html

There might be an opportunity to speed things up, using cython. In particular, I am thinking about ways to speed up both sl_signal and voxel2fiber, and any suggestions on that are most welcome. 

My implementation. Will move to `dipy.core.optimize` once #446 is merged