AP_AHRS: make the healthy() method const

libraries/AP_AHRS/AP_AHRS.h

@@ -367,7 +367,7 @@ public:
     uint8_t get_active_accel_instance(void) const { return _active_accel_instance; }
 
     // is the AHRS subsystem healthy?
-    virtual bool healthy(void) = 0;
+    virtual bool healthy(void) const = 0;
 
     // true if the AHRS has completed initialisation
     virtual bool initialised(void) const { return true; };

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -960,7 +960,7 @@ void AP_AHRS_DCM::set_home(const Location &loc)
 /*
   check if the AHRS subsystem is healthy
 */
-bool AP_AHRS_DCM::healthy(void)
+bool AP_AHRS_DCM::healthy(void) const
 {
     // consider ourselves healthy if there have been no failures for 5 seconds
     return (_last_failure_ms == 0 || hal.scheduler->millis() - _last_failure_ms > 5000);

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -111,7 +111,7 @@ public:
     void estimate_wind(void);
 
     // is the AHRS subsystem healthy?
-    bool healthy(void);
+    bool healthy(void) const;
 
 private:
     float _ki;

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -300,7 +300,7 @@ bool AP_AHRS_NavEKF::using_EKF(void) const
 /*
   check if the AHRS subsystem is healthy
 */
-bool AP_AHRS_NavEKF::healthy(void)
+bool AP_AHRS_NavEKF::healthy(void) const
 {
     if (_ekf_use) {
         return ekf_started && EKF.healthy();

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -113,7 +113,7 @@ public:
     void set_ekf_use(bool setting) { _ekf_use.set(setting); }
 
     // is the AHRS subsystem healthy?
-    bool healthy(void);
+    bool healthy(void) const;
 
     // true if the AHRS has completed initialisation
     bool initialised(void) const;