AP_AHRS: make get_position() const

This allows use from within AP_Mission

libraries/AP_AHRS/AP_AHRS.h

@@ -202,7 +202,7 @@ public:
 
     // get our current position estimate. Return true if a position is available,
     // otherwise false. This call fills in lat, lng and alt
-    virtual bool get_position(struct Location &loc) = 0;
+    virtual bool get_position(struct Location &loc) const = 0;
 
     // return a wind estimation vector, in m/s
     virtual Vector3f wind_estimate(void) = 0;

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -872,7 +872,7 @@ float AP_AHRS_DCM::get_error_yaw(void)
 
 // return our current position estimate using
 // dead-reckoning or GPS
-bool AP_AHRS_DCM::get_position(struct Location &loc)
+bool AP_AHRS_DCM::get_position(struct Location &loc) const
 {
     loc.lat = _last_lat;
     loc.lng = _last_lng;

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -83,7 +83,7 @@ public:
     void reset_attitude(const float &roll, const float &pitch, const float &yaw);
 
     // dead-reckoning support
-    virtual bool get_position(struct Location &loc);
+    virtual bool get_position(struct Location &loc) const;
 
     // status reporting
     float           get_error_rp(void);

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -129,7 +129,7 @@ void AP_AHRS_NavEKF::reset_attitude(const float &_roll, const float &_pitch, con
 }
 
 // dead-reckoning support
-bool AP_AHRS_NavEKF::get_position(struct Location &loc)
+bool AP_AHRS_NavEKF::get_position(struct Location &loc) const
 {
     if (using_EKF() && EKF.getLLH(loc)) {
         return true;

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -56,7 +56,7 @@ public:
     void reset_attitude(const float &roll, const float &pitch, const float &yaw);
 
     // dead-reckoning support
-    bool get_position(struct Location &loc);
+    bool get_position(struct Location &loc) const;
 
     // status reporting of estimated error
     float           get_error_rp(void);