approaching_time for rotorcrafts is implemented

Approaching time argument is added to the nav_approaching_from function. 
Notes:
1) Horizontal speed vector is used to check the approaching to a wp, rather than using the norm of the speed vector like in fixedwing firmware.
2) Negative approaching_time values are ignored (fly after destination waypoint case in fixedwing)
3) Default approaching time value (CARROT) set to 0 which means returning TRUE only if the rotorcraft is in proximity circle or crosses the normal line(for segment flights)

With this configuration 'nav_approaching_from' functions returns TRUE if;
1) Rotorcraft is in proximity circle (3m hard coded) 
2) Rotorcraft crosses the normal line in segment flight if 'approaching_time' argument is not presented in flight plan
3) Rotorcraft will be in proximity circle or will cross the normal line(for segment flight) in 'approaching_time' seconds presented in flight plan