Use a more robust parallelism detection

xs/xsp/Line.xsp

@@ -25,6 +25,9 @@
     double length();
     double atan2_();
     double direction();
+    bool parallel_to(double angle);
+    bool parallel_to_line(Line* line)
+        %code{% RETVAL = THIS->parallel_to(*line); %};
     Point* midpoint();
     Clone<Point> point_at(double distance);
     Polyline* as_polyline()