Retrieve bibliographic data

[joeroe]

IntChron data records are associated with bibliographic references in a few ways:

1. The records themselves contain a list of references in structured form (JSON format) or plaintext (CSV and TSV formats).
2. Requesting a record in .bib format returns all the associated references in BibTeX format.
3. Records also contain keys which refer to 'ref' or 'doi' records (https://intchron.org/ref).

We should use these. One option would simply be to pull the full references with the records as either text or BibTex. But I think it would be more robust to retrieve just the keys and later use these to get BibTeX from the corresponding 'ref' records, perhaps with a separate function.