[magnetometer] Should we expose heading?

anssiko has just created a new issue for 
https://github.com/w3c/magnetometer:

== Should we expose heading? ==
[ Spun off from 
https://github.com/w3c/magnetometer/issues/3#issuecomment-248446835 ]

There seems to be a rather strong use case to expose `heading`. For 
example, J5 does not expose raw `xyz` magnetometer data, but `heading`
 (in degrees).

Per https://w3c.github.io/sensors/#high-vs-low-level that would 
warrant its own high-level API.

Please view or discuss this issue at 
https://github.com/w3c/magnetometer/issues/14 using your GitHub 
account