Coordsystem default dimension and whether it should be there

[neuromechanist]

Looking at HBN-EEG data (both the older ds004186 and the newer ds005505), they both have electrodes.tsv and coordsystem.json there. I think we discussed that the two files should not be there when there is no individual digitization data.

I will remove these two files from the HBN-EEG datasets manually.

The other issue is that the default units in the coordsystem.json is mentioned as mm, which might not be the case in all instances. HBN data, for example, is apparently in cm.