ENH: Expanding functionality for mixed source spaces and source estimates

This call to ```_compute_nearest``` takes 13 seconds when it compares the whole mri volume grid to the surface (or discrete) vertex positions. So it will take at least 26 seconds for both hemispheres.

The purpose here is to find the nearest mri voxel for each vertex. Is there a faster way to do this than ```_compute_nearest``` (which uses ```ball_tree```)?