docs: def enforce_periodic_boundary_conditions( self ):
          """
          Ensure that all lattice sites are within the central periodic image of the simulation cell.
          Sites that are outside the central simulation cell are mapped back into this cell.
          
          Args:
              None
  
          Returns:
              None
          """
          for s in self.sites:
              for i in range(3):
                  if s.r[i] < 0.0:
                      s.r[i] += self.cell_lengths[i]
                  if s.r[i] > self.cell_lengths[i]:
                      s.r[i] -= self.cell_lengths[i]