Exporting surface sources as NIfTI volume

[drammock (Dan McCloy)]

@christian-oreilly

I could used this code to implement a BaseSourceEstimate.as_volume and submit a PR for that. I cannot commit to preparing a test that compares it with the output of mri_surf2vol though. I can check it, but in my experience these things that seem simple have a way to end up taking much more time than initially expected and I have limited time I can put on this.

This is still helpful! If one of us needs to take over the PR to get it thoroughly tested, that is still fine. Many hands make for light work, as they say.