code: def append(self, species, coords, coords_are_cartesian=False,
                 validate_proximity=False, properties=None):
          
          return self.insert(len(self), species, coords,
                             coords_are_cartesian=coords_are_cartesian,
                             validate_proximity=validate_proximity,
                             properties=properties)
docs: Append a site to the structure.
  
          Args:
              species: Species of inserted site
              coords (3x1 array): Coordinates of inserted site
              coords_are_cartesian (bool): Whether coordinates are cartesian.
                  Defaults to False.
              validate_proximity (bool): Whether to check if inserted site is
                  too close to an existing site. Defaults to False.
              properties (dict): Properties of the site.
  
          Returns:
              New structure with inserted site.