Add LitCovid topics as topicCategory

[flaneuse]

LitCovid has already classified the publications into various topics; we'd like to store this info as topicCategory. Unfortunately (sorry!) I just figured out that doesn't seem to be in any of their .json dumps BUT it does turn out that they have a special COVID-only API. Might be worthwhile to use that API instead of the .json dumps, but definitely not as important as getting the API up and running.

https://www.ncbi.nlm.nih.gov/research/coronavirus-api/search/
https://www.ncbi.nlm.nih.gov/research/coronavirus-api/publication/32323499

[gtsueng]

I wrote a quick parser to just grab the topicCategories. The BioThings should be able to seamlessly merge the results of the litcovid parser and the litcovidtopics parser given that they both use the same _ids and there is no overlapping properties other than the _ids (so there are no conflicts to resolve).