Center control for primitives.

A default of center=true on all primitives can make it very difficult to build complex objects. It is much better to have the choice. This would break backward compatibility, as the default is false, as it is in Scad. I have thought about how to get around breaking backward compatibility, but I find all the solutions I have come up with distasteful.  Looking through lots of code, I see that center=true is far less common than the default of false.