WIP: Find the closest vertex on a sphere for an input vector.

This is useful for both https://github.com/nipy/dipy/pull/460 and https://github.com/nipy/dipy/pull/419 so needs to take precedence. It's not quite done yet, though. Will hopefully have a chance to finish this later today. 
I also have a similar function like that, and by experience not using real unit vectors (like bvecs for example, which are almost unit vector by default) let to instability in the result.

Why not make it trulty general by dividing by the l2 norm and be able to pass any vector as an added bonus? I also clip the result between -1 and 1 because of imprecision in the division step.