SGTime: use SGGeod, quieter init.

It's common to use SGTime to calculate the the Julian Date, without providing time-zone information. Don't warn in these cases, since the behaviour is fine.

simgear/math/SGGeod.hxx

@@ -82,9 +82,12 @@ public:
   /// Set the geodetic elevation from the argument given in feet
   void setElevationFt(double elevation);
 
-  // Compare two geodetic positions for equality
+  /// Compare two geodetic positions for equality
   bool operator == ( const SGGeod & other ) const;
 
+  /// check the Geod contains sane values (finite, inside appropriate
+  /// ranges for lat/lon)
+  bool isValid() const;
 private:
   /// This one is private since construction is not unique if you do
   /// not know the units of the arguments. Use the factory methods for
@@ -345,6 +348,20 @@ SGGeod::operator == ( const SGGeod & other ) const
          _elevation == other._elevation;
 }
 
+inline
+bool
+SGGeod::isValid() const
+{
+  if (std::isnan(_lon) || std::isnan(_lat)) return false;
+#ifdef SG_GEOD_NATIVE_DEGREE
+  return (_lon >= -180.0) && (_lon <= 180.0) &&
+  (_lat >= -90.0) && (_lat <= 90.0);
+#else
+  return (_lon >= -SGD_PI) && (_lon <= SGD_PI) &&
+  (_lat >= -SGD_PI_2) && (_lat <= SGD_PI_2);
+#endif
+}
+
 /// Output to an ostream
 template<typename char_type, typename traits_type>
 inline