code: def inside_cell( self, r ):
          
          centre = np.array( [ 0.5, 0.5, 0.5 ] )
          new_r = self.nearest_image( centre, r )
          return new_r
docs: Given a fractional-coordinate, if this lies outside the cell return the equivalent point inside the cell.
  
          Args:
              r (np.array): Fractional coordinates of a point (this may be outside the cell boundaries).
  
          Returns:
              (np.array): Fractional coordinates of an equivalent point, inside the cell boundaries.