def inside_cell( self, r ):
        """
        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.
        """
        centre = np.array( [ 0.5, 0.5, 0.5 ] )
        new_r = self.nearest_image( centre, r )
        return new_r
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.