Re: [magnetometer] Why magneticFieldX instead of x?

@marcoscaceres sorry I missed the original post. `Compass` & 
`Magnetometer` in J5 don't expose raw xyz magnetometer data (though 
they surely could), because generally the developer really just wants 
a heading in degrees, which is be computed from x and y.

```
let heading = Math.atan2(y, x);
let pi_2 = Math.PI * 2;

if (heading < 0) {
  heading += pi_2;
}

if (heading > pi_2) {
  heading -= pi_2;
}

let degrees = heading * (180 / Math.PI);
```

I view that as an undue burden on the developer. For true-north 
heading, just subtract a declination  angle. 


The **z** in x, y, z is used for computing the earth's magnetic 
field—I've written a robot that can navigate from any lat/lon point to
 any _other_ lat/lon point using a gps and compass/magnetometer—no 
**z** necessary. 


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