code: def set_number_of_atoms( self, n, selected_sites=None ):
          
          self.number_of_atoms = n
          self.atoms = species.Species( self.lattice.populate_sites( self.number_of_atoms, selected_sites=selected_sites ) )
docs: Set the number of atoms for the simulation, and populate the simulation lattice.
  
          Args:
              n (Int): Number of atoms for this simulation.
              selected_sites (:obj:(List|Set|String), optional): Selects a subset of site types to be populated with atoms. Defaults to None.
  
          Returns:
              None