Fix orientation & caging, add indicators & G Meter

Changes:

1. Sensor orientation now completes when sensors are mounted in any orientation.
   Previously when sensors were mounted "upside down" relative to my prototype board the orientation was rejected.
2. Only one cage process at a time is allowed.
   Previously if a user pressed "Cage" a second time while the algorithm was still calibrating from the first press,
   a second calibration would take place immediately, making it seem like it would never complete.
3. AHRS status lights were added to show whether the AHRS algorithm is using GPS, Attitude sensors (MPU9250), and/or
   Altitude sensors (BMP280), and whether it is currently Calibrating and/or Logging (which can eat up disk space quickly).
   It is especially important for the user to be able to see the calibration process occurring, so that it is apparent why the AI isn't responding.
4. A G Meter is shown on the GPS/AHRS page.
5. Magnetic Heading, Turn Rate, Slip/Skid Angle, and G Load are shown in text on the GPS/AHRS page.
   Magnetic Heading will not be useful until the magnetometer is calibrated.
   I may add an auto-calibration routine to a future release so that it will self-calibrate over the course of several flights, and/or add a manual calibration routine.
6. The slip/skid rectangle on the attitude indicator works now.
   For now, it will only indicate correctly if the sensor is level in coordinated flight.
   In the future, auto-calibration will do this for you automatically.
7. A few labels were changed in the UI to make meanings clearer to the casual user.
8. Deeper changes were made to the MPU9250 driver to improve its efficiency.
   One of these changes involved removing a rudimentary accelerometer calibration.
   As a result, when on the desktop, the AHRS will not quite indicate level--it will indicate the actual orientation of the sensor.
   Even fairly rigidly mounted boards may indicate a degree or two of pitch or bank when sitting on a level desk.
   For now, you can regard this as an assist in manually leveling your stratux on your glareshield.
   An upcoming release will include a proper calibration, which is necessary to provide better pitch estimation.

Fix overturning in roll, better initialization, various UI improvements

Fixed various issues:

• Fix tendency to display overturning
• Orientation now fits on small (phone) screens
• Initialization more robust: less sensitive to glareshield vibrations and turbulence
• Separate UI switches for enabling IMU and BMP sensors
• UI switch to enable csv logging of AHRS data
• AHRS csv logging stops if disk fills to > 95%
• Logs go to correct location for FlightBox
• Various Go interface changes

THIS SOFTWARE IS STILL BETA. Feedback is very much appreciated.

Beta AHRS for Stratux - fixes some panics

This is the second public release of a beta AHRS for stratux. It requires an MPU-9250 sensor.

The algorithm calculates a basic attitude from the GPS flight track and then augments this with information from the sensor to provide a more responsive, smoother and more accurate estimate of the aircraft's attitude.

See https://westphae.github.com/stratux for more information.

Changes:

1. Fixed an issue where if the Sensors switch were on and no BMP280 were attached, gen_gdl90 would panic.
2. Fixed a panic in network.go due to concurrent reading/writing of a map.

Initial public AHRS release

This is the first public release of a beta AHRS for stratux. It requires an MPU-9250 sensor.

The algorithm calculates a basic attitude from the GPS flight track and then augments this with information from the sensor to provide a more responsive, smoother and more accurate estimate of the aircraft's attitude.

See https://westphae.github.com/stratux for more information.

NOTE: gen_gdl90 was compiled in this release with Go1.7. I am seeing panics due to concurrent read/writes from various maps in network.go, which apparently result when code is compiled with Go1.6 or greater. I am testing when compiled with Go 1.5.4 and will issue a new release when able.