To be consistent with `do_forward_solution`, I think we should make it so `fname=None` does not output a file at all, with `fname=True` using the MNE default name. I like having these in one function because it seems simpler to me, more consistent with `do_forward_solution`, and eventually less cumbersome on users (I imagine most will just create and write as opposed to doing anything with it in Python, so making this a two-step process increases work for users), but I'm okay with changing it for clarity's sake. Let me know...