Root coordinate system singleton is not unique with multiple translation units

There must be only one root coordinate system. It can be queried via the get_root_CoordinateSystem() function, which implements a singleton pattern. It must not be declared static since multiple translations units will have their own copies, which is what we want to avoid. This currently happens in testModules, which is built from several object files.

(note that it is currently not really an issue to actually have multiple roots; everything still works as it should)