Definition of the `_cell_wave_vector.seq_id` data item

[vaitkus]

The _cell_wave_vector.seq_id data item is defined an Integer in the range of 0:. However, items _atom_site_Fourier_wave_vector.q1_coeff_seq_id, ..., _atom_site_Fourier_wave_vector.q8_coeff_seq_id that link to it are instead defined as Code with the range of 1:8. Three questions that arise from this:

1. Should the type be Integer or Code (e.g. having a numeric value might be important to some widely used notation).
2. The more restrictive range of 1:8 technically does not conflict with the wider 0: range. I should probably update the checker to allow child items to map to only a subset of the possible parent values?
3. Can Code items even be assigned numeric ranges?

[vaitkus]

The issue has been resolved by merging PR #30 from this repository and PR COMCIFS/cif_core#564 from the cif_core repository.

All these items are now consistently defined as integers with the enumeration range of 1:.