[ins] reset reference position from INS not from NAV

Nav is (almost) independent of the GPS which is a sensor and should only
be seen by the INS/AHRS subsystems.
It also allow to make shared INS filters more easily (example with
invariant filter).
[ins] cleaning and dox update to better match what it actually does
[ins] remove hf|vf_realign from global ins struct

and fix some missing fuctions
[ins] remove ins_realign_v and ins_realign_h
[modules] remove obsolete cam_track
[ins] use weak functions for things that don't need to be implemented by each algorithm
[ins] vf cleanup: struct for states
[ins] minor hff cleanup
Merge pull request #644 from paparazzi/ins_reset

- reset reference position from INS not from NAV
- remove hf_realign, vf_realign from ins struct
- remove ins_realign_v and ins_realign_h API functions
- weak functions for things that don't need to be implemented in each algoritm