change the way to get the txaonpath and the names files

[chenyian-nibio]

To ensure the taxonomy data could be updated, I changed how to create the necessary taxonpath.json and names.json files.

The source will be fetched from NCBI, and processed to the product files. In case the user would like to use a specific version of database contents, just put the files (names.dmp and nodes.dmp) or create symbolic links in the ./db folder.