getNamesAndNodes: Download names and nodes files from NCBI
Description
Download a taxdump.tar.gz file from NCBI servers and extract the names.dmp and nodes.dmp files from it. These can then be used to create a SQLite database with read.names.sql and read.nodes.sql. Note that if the files already exist in the target directory then this function will not redownload them. Delete the files if a fresh download is desired.