Returns a data.frame that lists the available species strings and their taxonomy Ids.
NCBI maintains a collection of unique taxonomy Ids and pairs these with associated genus and species designations. This function returns the set of pre-processed values that we use to check that something is a valid Taxonomy ID (or species name)
A data frame with 1 row per species designation and two columns. The 1st column is the taxonomy Id. The second columns is the species name.
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