AP_AHRS: added have_ekf_logging() API

libraries/AP_AHRS/AP_AHRS.h

@@ -188,6 +188,11 @@ public:
         return nullptr;
     }
 
+    // is the EKF backend doing its own sensor logging?
+    virtual bool have_ekf_logging(void) const {
+        return false;
+    }
+    
     // Euler angles (radians)
     float roll;
     float pitch;

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -1272,5 +1272,11 @@ bool AP_AHRS_NavEKF::getGpsGlitchStatus()
 }
 
 
+// is the EKF backend doing its own sensor logging?
+bool AP_AHRS_NavEKF::have_ekf_logging(void) const
+{
+    return false;
+}
+
 #endif // AP_AHRS_NAVEKF_AVAILABLE
 

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -220,6 +220,9 @@ public:
     // used by Replay to force start at right timestamp
     void force_ekf_start(void) { force_ekf = true; }
 
+    // is the EKF backend doing its own sensor logging?
+    bool have_ekf_logging(void) const override;
+    
 private:
     enum EKF_TYPE {EKF_TYPE_NONE=0,
 #if AP_AHRS_WITH_EKF1