Copter/Rover: outstanding items for long distance journeys

The "Extereme ArduPilot" project resolved the majority of issues that caused a vehicle to lose position estimation or control accuracy if it travelled far from the EKF origin. This is a list of known outstanding items that should still be resolved:

- AHRS methods that should use ptype
- get_relative_position_NED_home
- get_relative_position_NED_origin
- get_relative_position_NE_home
- get_relative_position_NE_origin
- get_relative_position_D_home?
- get_relative_position_D_origin?
- writeExtNavData
- AP_Math::SCurve
- calculate_track
- move_from_pos_vel_accel
- move_to_pos_vel_accel
- get_track
- set_kinematic_limits
- segment[]::end_pos
- track
- AP_SmartRTL:
- get_point, pop_point, peek_point
- set_home
- update
- _path, prune_loop_t.midpoint

Start by tracing the long-distance EKF-origin handling around the listed AHRS methods, AP_Math::SCurve methods, and AP_SmartRTL methods. Determine the required ptype and position-handling changes across those subsystems; done means the outstanding methods no longer lose position-estimation or control accuracy during long-distance journeys.

Written by the indexing model from the issue text.