*_channels.tsv: "units" column should be "unit"

[hoechenberger]

In *_channels.tsv for MEG, EEG, and iEEG data, all column names except for software_filters and units are in the singular form. software_filters being plural does make sense because one may specify multiple filters per channel.

For units, however, I think the current specification introduces an inconsistency: to my knowledge, only a single measurement unit can be reasonably assigned to any given channel.

I would therefore propose to deprecate the units column in favor of a unit column.

Screenshot of the MEG specs:

[sappelhoff]

I brought up some time in 2018 or 2019, but it was always deemed low impact and annoying, so we skipped it. Tracked here: bids-standard/bids-2-devel#15

[hoechenberger]

Whoops, I must've missed this when searching existing issues. Closing as a dupe.