Cell files with lattice_abc vectors were broken, and this was not
causing test failures because it wasn't tested. Here we:

- Create test files with and without explicit dimension units
- fix the treatment of lattice_abc data in "two-rows" format (no unit)
- support recent versions of Pymatgen by migrating to
- Lattice.from_parameters
  from the deprecated Lattice.from_lengths_and_angles
- Note that Lattice.from_parameters does not produce the same cell
  matrices as CASTEP, and raise a warning if the user has
  abs_positions. The positions will very likely be wrong in this case;
  but not many features of Sumo are impacted.

CASTEP and Pymatgen orientation are likely to be different from the
same lattice vectors. To avoid building incorrect structures, refuse
to work with lattice_abc and positions_abs simultaneously.