AP_Module: added external module hook library

this allows for external modules to be called at defined hook
locations in ArduPilot. The initial example is a module that consumes
the AHRS state, but this can be generalised to a wide variety of hooks

libraries/AP_Module/AP_Module_Structures.h

@@ -0,0 +1,100 @@
+/// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+/*
+  this defines data structures for public module interfaces in
+  ArduPilot. 
+
+  These structures are designed to not depend on other headers inside
+  ArduPilot, although they do depend on the general ABI of the
+  platform, and thus can depend on compilation options to some extent
+ */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stdint.h>
+#include <stdbool.h>
+
+#define AHRS_state_version 1
+
+enum AHRS_status {
+    AHRS_STATUS_INITIALISING    = 0,
+    AHRS_STATUS_UNHEALTHY       = 1,
+    AHRS_STATUS_HEALTHY         = 2
+};
+
+/*
+  export the attitude and position of the vehicle.
+ */
+struct AHRS_state {
+    // version of this structure (AHRS_state_version)
+    uint32_t structure_version;
+
+    // time since boot in microseconds
+    uint64_t time_us;
+    
+    // status of AHRS solution
+    enum AHRS_status status;
+    
+    // quaternion attitude, first element is length scalar. Same
+    // conventions as AP_Math/quaternion.h
+    float quat[4];
+
+    // euler angles in radians. Order is roll, pitch, yaw
+    float eulers[3];
+    
+    // global origin
+    struct {
+        // true when origin has been initialised with a global position
+        bool initialised;
+
+        // latitude and longitude in degrees * 10^7 (approx 1cm resolution)
+        int32_t latitude;
+        int32_t longitude;
+
+        // altitude AMSL in meters, positive up
+        float altitude;
+    } origin;
+
+    // global position
+    struct {
+        // true when we have a global position
+        bool available;
+
+        // latitude and longitude in degrees * 10^7 (approx 1cm resolution)
+        int32_t latitude;
+        int32_t longitude;
+
+        // altitude AMSL in meters, positive up
+        float altitude;
+    } position;
+    
+    // NED relative position in meters. Relative to origin
+    float relative_position[3];
+
+    // current rotational rates in radians/second in body frame
+    // order is roll, pitch, yaw
+    float gyro_rate[3];
+
+    // current earth frame acceleration estimate, including
+    // gravitational forces, m/s/s order is NED
+    float accel_ef[3];
+};
+
+
+/*
+  prototypes for hook functions
+ */
+typedef void (*hook_setup_start_fn_t)(uint64_t);
+void hook_setup_start(uint64_t time_us);
+    
+typedef void (*hook_setup_complete_fn_t)(uint64_t);
+void hook_setup_complete(uint64_t time_us);
+    
+typedef void (*hook_AHRS_update_fn_t)(const struct AHRS_state *);
+void hook_AHRS_update(const struct AHRS_state *state);
+
+#ifdef __cplusplus
+}
+#endif
+