Simplify_slices rewritten to C++, parallelized.

Added some move methods to Surface class.

xs/xsp/Print.xsp

@@ -113,6 +113,7 @@ _constant()
 
     void _slice();
     std::string _fix_slicing_errors();
+    void _simplify_slices(double distance);
     void detect_surfaces_type();
     void process_external_surfaces();
     void discover_vertical_shells();