I think the `meg_solution_grad` refers to this option:
```
grad : bool
    If True, compute the gradient of the field with respect to the dipole coordinates as well (Default: False).
```
Thus I don't think this has to do with mag+grad channel types, so much as the gradient of the lead field. Is that correct? I've never dealt with a `sol_grad` before, but based on how it's treated in `read_forward_solution`, this is also my sense of it.

Should I make a test case for it? That would settle it once and for all.