SGGeod: add static constructor of an invalid Geod

Use this to allow explicitly initializing a value which isValid
will return false for.

simgear/math/SGGeod.hxx

@@ -29,6 +29,13 @@ public:
   /// Default constructor, initializes the instance to lat = lon = elev = 0
   SGGeod(void);
 
+  /**
+    return an SGGeod for which isValid() returns false.
+    This is necessaerby becuase for historical reasons, ther defaulrt constructor above initialsies to zero,zero,zero
+    which *is*
+   */
+  static SGGeod invalid();
+
   /// Factory from angular values in radians and elevation is 0
   static SGGeod fromRad(double lon, double lat);
   /// Factory from angular values in degrees and elevation is 0

simgear/math/SGGeodesy.cxx

@@ -663,3 +663,8 @@ SGGeodesy::radialIntersection(const SGGeod& a, double aRadial,
   result = SGGeod::fromGeoc(r);
   return true;
 }
+
+SGGeod SGGeod::invalid()
+{
+    return SGGeod::fromDeg(-999.9, -999.0);
+}