Input and output formats

We should be able to read and write data to/from these formats:

- [ ] Complex tables like @tsdye / `hm`: http://www.tsdye.online/harris-matrix/data-tables/
- [ ] Complex tables like `ArkMatrix`: https://gitlab.com/arklab/ArkMatrix/blob/master/docs/format_csv.md
- [ ] INTCHRON JSON files: https://doi.org/10.1017/RDC.2019.21 → https://github.com/joeroe/RIntChron
- [x] Generic tables of relations (i.e. "above", "below", "equal", as on a typical context sheet)
- [x] `BASP`/`ArchEd`/`Stratify` .LST files: http://archaeologic.al/wiki/Harris_Matrix#LST
- [ ] Formats used or exported by AISes: Field, Arches, ArcheoBase, ...?

We probably won't be handle these formats:

* .HM files from `ArchEd` – are in binary format, and the only software that could write/read them doesn't appear to be functional any more (see http://archaeologic.al/wiki/Harris_Matrix#ArchEd)
* CRMarchaeo/CIDOC CRM ontologies: http://cidoc-crm.org/crmarchaeo/sites/default/files/CRMarchaeo_v1.3.pdf – extremely complicated so indefinitely postponed pending evidence that anyone actually uses it.

No implementation files or tests are named. Start with the format checklist and the already supported generic relation tables and BASP/ArchEd/Stratify .LST files, then select one unchecked format for investigation. Done would mean a defined, working read and write path for that format, rather than the entire broad list.

Written by the indexing model from the issue text.