The way our current code works, I think we are guaranteed to be in `Am`.

In the C code, whether it's current or current density is determined by the `--csd` command-line option in `make_inverse_operator.c` (turning into `is_csd`), which in turn calls `mne_read_process_forward_solution` in `mne_read_process_forward_solution.c` (setting the variable `do_csd`). The equivalent in our code would need to be in `forward.py` in `read_forward_solution`, which has no option in it for computing `csd`. I vaguely recall seeing that option when I was doing the inverse operator tweaks a few months back. I figured we didn't include the `csd` option because there had been no demand for it...

@agramfort can you double-check this makes sense? It would be too bad to get the units wrong.