Retrieve bibliographic data

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.
2. Records also contain keys which refer to 'ref' or 'doi' records ().

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.