added a lt operator to the header

added a lt operator to the header, so that I can use aiVectors as stl keys and such

include/assimp/vector3.h

@@ -88,6 +88,7 @@ public:
 	// comparison
 	bool operator== (const aiVector3t& other) const;
 	bool operator!= (const aiVector3t& other) const;
+	bool operator < (const aiVector3t& other) const;
 
 	template <typename TOther>
 	operator aiVector3t<TOther> () const;