get_metadata_from_one_pmid
                        Downloads metadata from Pubmed API for a single
                        provided PMID and exports
get_metadata_from_pmids
                        Downloads metadata from Pubmed API for a column
                        of PMID's in a data frame
get_pmids_from_one_search
                        Returns a list of PMID's for a provided Pubmed
                        search
get_pmids_from_searches
                        Downloads PMID results for a column of Pubmed
                        search queries in a data frame
intersection_check      Checks a column of PMID's for whether or not
                        they would appear in a Pubmed search result.
