Fetch position from GLOBAL_POSITION_INT to get timestamped positions

Getters removed because these data should be fetch through position array.

Use position messages instead of gps ones in case of gps denial.

Now `setTargetCoordinates` always go through the desired point by updating
regulary the projected coordinates.
Function `loiter` is now defined for the purpose where `setTargetCoordinates`
was called with `going_through` parameter set to `false` (to loiter around a
point without going through it).