AP_Terrain: add static create method

libraries/AP_Terrain/AP_Terrain.h

@@ -74,19 +74,28 @@
   file:        first entries increase east, then north
  */
 
-class AP_Terrain
-{
+class AP_Terrain {
 public:
-    AP_Terrain(AP_AHRS &_ahrs, const AP_Mission &_mission, const AP_Rally &_rally);
-
     enum TerrainStatus {
         TerrainStatusDisabled  = 0, // not enabled
         TerrainStatusUnhealthy = 1, // no terrain data for current location
         TerrainStatusOK        = 2  // terrain data available
     };
 
+    static AP_Terrain create(AP_AHRS &_ahrs,
+                             const AP_Mission &_mission,
+                             const AP_Rally &_rally) {
+        return AP_Terrain{_ahrs, _mission, _rally};
+    }
+
+    constexpr AP_Terrain(AP_Terrain &&other) = default;
+
+    /* Do not allow copies */
+    AP_Terrain(const AP_Terrain &other) = delete;
+    AP_Terrain &operator=(const AP_Terrain&) = delete;
+
     static const struct AP_Param::GroupInfo var_info[];
-    
+
     // update terrain state. Should be called at 1Hz or more
     void update(void);
 
@@ -175,6 +184,8 @@ public:
     void get_statistics(uint16_t &pending, uint16_t &loaded);
 
 private:
+    AP_Terrain(AP_AHRS &_ahrs, const AP_Mission &_mission, const AP_Rally &_rally);
+
     // allocate the terrain subsystem data
     bool allocate(void);