Don't arrange the objects on the build plate. The model coordinates
define the absolute positions on the build plate.
The option --print-center will be ignored.
Also there is a fix in the command line invocation of --duplicate-grid
so it will run, but still there is something broken there and the results
are not correct.
Some code copied from xs-model branch.
Also:
* Generate ::Ref classes programatically.
* Add separate __REGISTER_CLASS macro
(for use where forward declaration won't work, i.e. typedefs)
