code: def units(self):
          
          existing = [i.units for i in self.geometry.values()]
  
          if any(existing[0] != e for e in existing):
              # if all of our geometry doesn't have the same units already
              # this function will only do some hot nonsense
              raise ValueError('models in scene have inconsistent units!')
  
          return existing[0]
docs: Get the units for every model in the scene, and
          raise a ValueError if there are mixed units.
  
          Returns
          -----------
          units : str
            Units for every model in the scene