AHRS: make get_error_rp and get_error_yaw const

libraries/AP_AHRS/AP_AHRS.h

@@ -217,11 +217,11 @@ public:
 
     // return the average size of the roll/pitch error estimate
     // since last call
-    virtual float get_error_rp(void) = 0;
+    virtual float get_error_rp(void) const = 0;
 
     // return the average size of the yaw error estimate
     // since last call
-    virtual float get_error_yaw(void) = 0;
+    virtual float get_error_yaw(void) const = 0;
 
     // return a DCM rotation matrix representing our current
     // attitude

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -193,12 +193,12 @@ bool AP_AHRS_NavEKF::get_position(struct Location &loc) const
 }
 
 // status reporting of estimated errors
-float AP_AHRS_NavEKF::get_error_rp(void)
+float AP_AHRS_NavEKF::get_error_rp(void) const
 {
     return AP_AHRS_DCM::get_error_rp();
 }
 
-float AP_AHRS_NavEKF::get_error_yaw(void)
+float AP_AHRS_NavEKF::get_error_yaw(void) const
 {
     return AP_AHRS_DCM::get_error_yaw();
 }

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -64,8 +64,8 @@ public:
     bool get_position(struct Location &loc) const;
 
     // status reporting of estimated error
-    float           get_error_rp(void);
+    float           get_error_rp(void) const;
-    float           get_error_yaw(void);
+    float           get_error_yaw(void) const;
 
     // return a wind estimation vector, in m/s
     Vector3f wind_estimate(void);