'Return the unit of measurement of this entity, if any.'

@property
def unit_of_measurement(self):

    if (self._data_key == 'temperature'):
        return TEMP_CELSIUS
    elif (self._data_key == 'humidity'):
        return '%'
    elif (self._data_key == 'illumination'):
        return 'lm'
    elif (self._data_key == 'lux'):
        return 'lx'
    elif (self._data_key == 'pressure'):
        return 'hPa'
