code: def convert_units(self, desired, guess=False):
          
          # if there is no geometry do nothing
          if len(self.geometry) == 0:
              return self.copy()
  
          current = self.units
          if current is None:
              # will raise ValueError if not in metadata
              # and not allowed to guess
              current = units.units_from_metadata(self, guess=guess)
  
          # find the float conversion
          scale = units.unit_conversion(current=current,
                                        desired=desired)
  
          # exit early if our current units are the same as desired units
          if np.isclose(scale, 1.0):
              result = self.copy()
          else:
              result = self.scaled(scale=scale)
  
          # apply the units to every geometry of the scaled result
          result.units = desired
  
          return result
docs: If geometry has units defined convert them to new units.
  
          Returns a new scene with geometries and transforms scaled.
  
          Parameters
          ----------
          desired : str
            Desired final unit system: 'inches', 'mm', etc.
          guess : bool
            Is the converter allowed to guess scale when models
            don't have it specified in their metadata.
  
          Returns
          ----------
          scaled : trimesh.Scene
            Copy of scene with scaling applied and units set
            for every model