Mixed source space

Code to generate source spaces with cortical surfaces and sub-cortical volumes.
I'm -1 on using indices, it seems less explicit. Are there cases where indices would be useful?

Also, do we have a utility for listing all segmentation names in a `.mgz` file? If not, we should add it (with tests). It might just be a simple wrapper to `nibabel`, not sure. As a user it would be nice to be able to query `get_volume_names` or so to get this info, though. (@agramfort can probably think of a better name)