Update CDM Observation table, ETL conventions for a person's "history of event"

[MelaniePhilofsky]

The historical event for a person should be recorded in the Observation table using the following as guidance to properly populate the observation_concept_id field: (i) an all-in-one bucket History of event concept that implies the semantics of a historical event, context, qualifier, or value; (ii) a hierarchy of enhanced historical concepts that bring additional time context (History of event within... and History of event longer than...)." The historical event will be recorded in the value_as_concept_id field.

[MelaniePhilofsky]

This convention needs documentation. Moving it to the Themis GitHub.

• History of versus family history of
• Condition versus Observation table
• observation concept_id & value_as_concept_id
• Type concept_id
• STCM doesn't support 'maps to' and 'maps to value'
• Utilize C/CR
• Note on CDM documentation with link to Themis convention library