Allow use to specify altitude adjustment for geoid height diff

For devices without a barometer, altitude is calculated using GPS. GPS uses mean sea height (a uniform ellipse) to define altitude but the actual height of the sea level varies across the globe. This is a manual workaround for making the adjustment.

The code includes a GaggleConfig class which can be used to disable use of a barometer on devices that have them in order to test the fix without the barometer overwriting the GPS provided altitude.

I could add the disabling of the Barometer as a user preference if anyone wants it (battery conservation?)