A study_external_data object (which inherits from a list) which
contains some of the following.
doi, character, the DOI for the paper describing this study
external_data_url, character, a URL to an external data repository
(e.g. a treebase entry) if one exists.
pubmed_id character, the unique ID for this study in the NCBI's pubmed database
popset_ids character, vector of IDs for the NCBI's popset database
nucleotide_ids character, vector of IDs for the NCBI's nucleotide database