WIP FIX: Discrete source space with 

This *does not* work. All this does is expose the issue, namely that `src_mri_t` and `voxel_size` are not provided when a discrete source space is created, so creating the interpolation matrix fails. This may require refactoring, or maybe just defining variables appropriately.

I'm not sure when I'll get a chance to look at this issue, so someone else feel free to take a shot. @mshamalainen will presumably come up with a fix for the C code, in which case we can use that.