Importcenter

I am not sure if you will want this, but I thought I'd submit it as a possible solution for the "how do we import STL that have strange offsets" question that was discussed a few weeks ago by Emmett Lallish and others. 

This will add a 'center=true/false' option to the import() function, just like exists for sphere() and cube(). The imported STL will be 'centered' at the origin. 

It adds a 'translate' function to 'polyset' b/c it seems the easiest fastest way to do the transformation. ( i suppose inserting a transform() node above the import() might be another way, but ... it seemed complicated )