Refactor/improve representation of GKS Core Domain Entities (Disease, Phenotype, Condition)

[mbrush]

Modeling of these domain entities for the metakb-cvc implementation work defined separate classes for these concepts that make distinctions inconsistent with how the VA team has thought about and modeled Genetic Conditions more generally. e.g. metakb-cvc Conditions are specifically defined to necessarily include a collection of >1 Disease or Phenotype.

On the Aug 30 2022 VA call, there was consensus that this approach - while practical for the current metakb work, is semantically inconsistent with how these concepts are understood. We will return to refactor / align representations after the plenary/connect meetings.