Parse force constants part

        Physical unit of force cosntants in the file is Ry/au^2.

def _parse_fc(self, f, natom, dim):
        """Parse force constants part

        Physical unit of force cosntants in the file is Ry/au^2.

        """

        ndim = np.prod(dim)
        fc = np.zeros((natom, natom * ndim, 3, 3), dtype='double', order='C')
        for k, l, i, j in np.ndindex((3, 3, natom, natom)):
            line = f.readline()
            for i_dim in range(ndim):
                line = f.readline()
                # fc[i, j * ndim + i_dim, k, l] = float(line.split()[3])
                fc[j, i * ndim + i_dim, l, k] = float(line.split()[3])
        return fc