Works out the unit a channel's calibrated values are in.
Identity: always RAW_UNIT, whatever else is configured.
Otherwise the calibration's own unit when set.
Otherwise (configs written before calibrations had a unit) the channel's
measurement_units entry, unless it is blank or V: V was the value filled in
when nobody chose a unit, so it says nothing about a calibrated channel.
Works out the unit a channel's calibrated values are in.
unitwhen set.measurement_unitsentry, unless it is blank orV:Vwas the value filled in when nobody chose a unit, so it says nothing about a calibrated channel.