More clearly delineate what belongs in FHIR resources and what is added/derived

[laurendukes]

Right now our FHIR resource classes (Observation, RadiologyReport, etc.) are mostly what the FHIR DSTU2 spec says, but we've sprinkled in a few fields of our own. For better understandability and potential reuse, anything that we derive should get put in a superclass for those resources.

As part of this work, let's also look at all the fields we extract for each resource and ensure that we're handling them as the spec requires and not assuming that they come in one way or the other.

[hgui]

1. Potentially create parent class of DomainResource for Observations and DiagnosticReport
2. Extract requestID into a parent class to maintain clarity in FHIR standard