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've been using something like the following for this:

```
cos_sim = abs(np.dot(self.vertices, xyz))
return np.argmax(cos_sim)
```

 If this is being done repeatedly it might be worth building a KDtree for it.