mne-bids: [BUG] _channels_tsv writes default unit for channel type

mne_bids._channels_tsv writes a unit that is assumed based on the channel type:

https://github.com/mne-tools/mne-bids/blob/e03f013cfc897d116ce4d67d7bce6204d94a40a7/mne_bids/mne_bids.py#L89

This is problematic because different manufacturers save the raw data in different units. Sometimes there are even differences within single manufacturer data formats. Usually, the units are declared in the dataset - when reading the data with MNE-Python, the data gets automatically scaled to e.g., VOLTS so that all MNE-Python objects consistently work in VOLTS.

This is a problem, because we are then copying the non-mne-modified raw data to the bids directory … and the units will not match.