Get magnetometer working with a simple calibration

[westphae]

A proper mag calibration procedure is not trivial, but we can get most of the way there with a simple calibration to determine each axis' min/max values.

1. Remove mag cal params from driver, save in stratux.conf.
2. Heading Indicator UI: magnetic heading, heading bug, adjust knobs.
3. Calibration UI: show current min/max for each axis in a Vue bar type display, green/yellow/red.

Later:

1. Code geomagnetic model in Go to calculate correct magnetic field in any location.
2. Proper live calibration using Kalman filter approach.
3. CDI & GS on HI UI.
4. Calculate winds aloft given airspeed.