AP_Common: add Location::is_zero and ::zero member functions

libraries/AP_Common/Location.cpp

@@ -20,8 +20,7 @@ AP_Terrain *Location_Class::_terrain = NULL;
 /// constructors
 Location_Class::Location_Class()
 {
-    lat = lng = alt = 0;
-    options = 0;
+    zero();
 }
 
 Location_Class::Location_Class(int32_t latitude, int32_t longitude, int32_t alt_in_cm, ALT_FRAME frame)

libraries/AP_Common/Location.h

@@ -67,6 +67,10 @@ public:
     // extrapolate latitude/longitude given distances (in meters) north and east
     void offset(float ofs_north, float ofs_east);
 
+    bool is_zero(void) { return (lat == 0 && lng == 0 && alt == 0 && options == 0); }
+
+    void zero(void) { lat = lng = alt = 0; options = 0; }
+
 private:
     static const AP_AHRS_NavEKF *_ahrs;
     static AP_Terrain *_terrain;