Include OMOP mappings

[caufieldjh]

This is similar to KG-OBO, but not explicitly OBO, so it goes here.

As discussed in Feb 1 2022 Monarch Huddle, it would be useful to have readily-available OMOP terms for KG assembly, and these can be mapped using OMOP2OBO.

Will likely have to make a specific request for the pre-made mappings, or make from scratch if possible to ensure everything's up to date.

May require use of some UMLS resources like SNOMED that tend to create barriers to distribution.