Some private methods (relating to calibration/compensation) are no
longer exposed. In addition, the driver auto-detects the chip (BMP280
or BME280) and acts accordingly, rather than requiring the
specification of a chip id in the ctor.
The getHumidity() method no longer accepts an arguement representing
pressure at sea level. A new method is provided to specify this.
Signed-off-by: Jon Trulson <jtrulson@ics.com>
