James,
The scripts in examples/units (and run by backend_driver.py) are broken; they have not been updated to match your addition of the new mandatory axis argument to convert() and default_units(). Would you fix them, please? (While you are in the neighborhood, you might also update the methods by switching to the @staticmethod decorator.)
The argument was also missing from the units.py docstring, but I have fixed that.
Thank you.
Eric