navigator : add support for precision landing

src/modules/navigator/CMakeLists.txt

src/modules/navigator/mission.cpp

src/modules/navigator/mission.h

src/modules/navigator/navigation.h

src/modules/navigator/navigator.h

src/modules/navigator/navigator_main.cpp

src/modules/navigator/precland.cpp

src/modules/navigator/precland.h

src/modules/navigator/precland_params.c