move default AHRSFactor constructor to public for wrapper

gtsam/navigation/AHRSFactor.h

@@ -139,9 +139,6 @@ class GTSAM_EXPORT AHRSFactor: public NoiseModelFactorN<Rot3, Rot3, Vector3> {
 
   PreintegratedAhrsMeasurements _PIM_;
 
-  /** Default constructor - only use for serialization */
-  AHRSFactor() {}
-
 public:
 
   // Provide access to the Matrix& version of evaluateError:
@@ -154,6 +151,9 @@ public:
   typedef std::shared_ptr<AHRSFactor> shared_ptr;
 #endif
 
+  /** Default constructor - only use for serialization */
+  AHRSFactor() {}
+
   /**
    * Constructor
    * @param rot_i previous rot key