This function returns the turn angels of a great circle track.
## S4 method for signature '.MoveTrackSingle' turnAngleGc(x)
Move object, in long lat projection
On great circle tracks the bearing of arrival on a point is not the same as witch the previous point was left. This function returns the difference between these bearings between -180 and 180. The bearings are calculated using the functions bearing and finalBearing of the geosphere package.
The function returns the turning angles in degrees.