Exporting non overlapping STL files

[shimwell]

The export stl function loops through the volumes in reactor.solid and saves each volume to an stl file.

However there is a nice option to first merge the surfaces then export the stl files. This should result in non overlapping STL files. Non overlapping STL files are potentially more useful for neutron transport.

This would just call the existing merge surfaces command before exporting to STL files