Units before and after compute_current_source_density

[larsoner (Eric Larson)]

All data in MNE should be in SI units so the unit should be V/m^2 I think, and the code seems to suggest so, too:

mne-tools/mne-python/blob/2c696d5ca0bc48b44f7916322b0759c760c54f94/mne/preprocessing/_csd.py#L187