AP_AHRS: make some more functions const

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -272,7 +272,7 @@ AP_AHRS_DCM::_P_gain(float spin_rate)
 }
 
 // return true if we have and should use GPS
-bool AP_AHRS_DCM::have_gps(void)
+bool AP_AHRS_DCM::have_gps(void) const
 {
     if (!_gps || _gps->status() <= GPS::NO_FIX || !_gps_use) {
         return false;
@@ -281,7 +281,7 @@ bool AP_AHRS_DCM::have_gps(void)
 }
 
 // return true if we should use the compass for yaw correction
-bool AP_AHRS_DCM::use_compass(void)
+bool AP_AHRS_DCM::use_compass(void) const
 {
     if (!_compass || !_compass->use_for_yaw()) {
         // no compass available

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -60,7 +60,7 @@ public:
     // if we have an estimate
     bool airspeed_estimate(float *airspeed_ret);
 
-    bool            use_compass(void);
+    bool            use_compass(void) const;
 
 private:
     float _ki;
@@ -76,7 +76,7 @@ private:
     float           yaw_error_compass();
     void            euler_angles(void);
     void            estimate_wind(Vector3f &velocity);
-    bool            have_gps(void);
+    bool            have_gps(void) const;
 
     // primary representation of attitude
     Matrix3f _dcm_matrix;