Add support for Location.partOf

The FHIR spec supports a location hierarchy whereby one location can be indicated to be partOf another. OpenMRS has a very similar hierarchy through the `parentLocation` property. Any location with a parent location should advertise that parent location by setting the partOf property to a reference to the appropriate location. For an idea on how we construct references, please take a look at the PatientReferenceTranslatorImpl or EncounterReferenceTranslatorImpl classes.