docs: def centre_of_mass(self):
          """Returns the centre of mass of AMPAL object.
  
          Notes
          -----
          All atoms are included in calculation, call `centre_of_mass`
          manually if another selection is require.
  
          Returns
          -------
          centre_of_mass : numpy.array
              3D coordinate for the centre of mass.
          """
          elts = set([x.element for x in self.get_atoms()])
          masses_dict = {e: ELEMENT_DATA[e]['atomic mass'] for e in elts}
          points = [x._vector for x in self.get_atoms()]
          masses = [masses_dict[x.element] for x in self.get_atoms()]
          return centre_of_mass(points=points, masses=masses)