investigate PubTator as resource for NGD calculation

[andrewsu]

In #326, we implemented an NGD scoring system based on semmeddb. As a low priority item, we could also investigate creating an NGD service based on Pubtator, which presumably has significantly better performance on NER. Pubtator files are automatically updated for download at https://ftp.ncbi.nlm.nih.gov/pub/lu/PubTatorCentral/. We'd want to confirm that no one else in Translator had already done this.

[erikyao]

TODO: Consider implementing NGD using Painless (scripting on ES side)