create scheduled update of UMLS

[andrewsu]

UMLS is a key biomedical resource for genes and other entities. http://mygene.info/v3/metadata says the last update was April 2020. According to the release history, UMLS is updated twice a year every May and November. We should update UMLS to the latest version, and also have some sort of automated (or semi-automated) way to triggering these updates (or at least notifying a data manager). The relevant parser code is in https://github.com/biothings/mygene.info/tree/master/src/hub/dataload/sources/umls.

Related issues:

• https://github.com/biothings/mychem.info/issues/130
• https://github.com/biothings/mydisease.info/issues/46