create a remote magnetometer

[gautierhattenberger]

When using the remote GPS with indoor flight area, the heading is usually extracted from the GPS heading and it's usage is dependent of the INS/AHRS implementation, sometimes with invalid projections.
A better solution might be to compute a virtual measurement and project it on the body frame on the ground side. Then send it to the drone with a REMOTE_MAG message. The estimation filters will be able to use this as they would use a regular magnetometer when supported, instead of abusing the GPS heading field.