Clearer distinction between yaw & heading

Original issue reported here https://forum.geocommuns.fr/t/photos-360-avec-un-smartphone/1699/8?u=cayenne

Example image : https://panoramax.openstreetmap.fr/images/5c/d6/54/27/e89f-42b6-aefe-448372deea9f.jpg

The current version of tag reader only offer "heading" (GPS direction), but not the "yaw" (sphere correction on horizontal axis). We should add yaw in results, and make doc clearer about heading (it's just the GPS direction, nothing else).