TODO

-- Output a space group which is a factorized crystal symmetry group
   by the molecule symmetry group to the 'cif_molecule' and 'cif_p1'
   outputs.

++ Add options '--supercell' and '--supercell-span' to 'cif_molecule'
   and 'cif_p1'. Add option '--randomise-supercell' to pick random
   combinations of disorder groups.

   Alternatively, make a separate script 'cif_supercell' to generate
   supercells from the 'cif_molecule' and 'cif_p1' outputs. This will
   only be possible if a correct (factorized) symmetry group, not just
   P1, will be output by 'cif_molecule' and 'cif_p1'.

-- !!! Check that 'cif_filter' correctly outputs data stream of
   consistent single version (CIF1 or CIF2), even when presented with
   CIFs of different versions on the input.

-- Add option to 'cif_filter' to select which version of the CIF
   standard (CIF1 or CIF2) should be used for the output stream.