Function describeChannelUnit

Describes the unit of a channel's plotted values.

• Identity calibration: values are raw volts, so the unit is V. A configured unit other than V (possible only in older configs) is reported in warning, since the values are not in it.
• Any other calibration: the unit from resolveCalibrationUnit. Only an older config whose calibration has no unit and whose measurement_units entry is blank or V has none; the values are then labelled V and warning asks for the unit to be set in the LabJack config.