CORE MedicationRequest: provide simple access to "Category" and "Encounter"

[comorbidity]

https://github.com/smart-on-fhir/cumulus-library/blob/main/cumulus_library/studies/core/medication_request.sql

https://hl7.org/fhir/medicationrequest-definitions.html#MedicationRequest.category
https://hl7.org/fhir/medicationrequest-definitions.html#MedicationRequest.encounter

[comorbidity]

https://hl7.org/fhir/medicationrequest-definitions.html#MedicationRequest.category

Example usage

select distinct cat_coding.code as category_code
from
core__medicationrequest mr,
unnest(mr.category) AS t (mr_cat),
unnest(mr_cat.coding) AS t (cat_coding)

[dogversioning]

this is handled by the flattening table work done in #166