Calculate and set lattice parameters.

        Invoked by CELL_PARAMETERS tag.

def _set_lattice(self):
        """Calculate and set lattice parameters.

        Invoked by CELL_PARAMETERS tag.

        """

        unit = self._values[0]
        if unit == 'alat':
            if not self._tags['celldm(1)']:
                print("celldm(1) has to be specified when using alat.")
                sys.exit(1)
            else:
                factor = self._tags['celldm(1)']  # in Bohr
        elif unit == 'angstrom':
            factor = 1.0 / Bohr
        else:
            factor = 1.0

        if len(self._values[1:]) < 9:
            print("CELL_PARAMETERS is wrongly set.")
            sys.exit(1)

        lattice = np.reshape([float(x) for x in self._values[1:10]], (3, 3))
        self._tags['cell_parameters'] = lattice * factor