Mixed source space

Code to generate source spaces with cortical surfaces and sub-cortical volumes.
`, typically named `aseg.mgz` in the Freesurfer pipeline.`

In fact, it's always in the same place for each subject, right? Maybe we should support just passing `subject` instead. WDYT? It would be nice if we could do it for `mri` above, too. I'm starting to think we might want to just deprecate the old `setup_volume_source_space` function entirely and start with a nicer API that splits `volume` and `discrete`, and automates some of the path finding. But let's do that in another PR.