Define required/optional format(s) for OData

We need to define what format(s), if any, are required for a service to be OData "Conformant", and which are optional.

For example, we could say that JSON is required, and ATOM is optional, meaning a service that supported only ATOM would not be conformant. This would help clients but might prevent some services from being compliant. Also, if we added another format (say, RDF), it would prevent a service from implementing just RDF and claiming conformance.

We could, fore example, say that a service SHOULD support JSON for maximum interoperability, but not make it required. Or perhaps we can define levels of conformance.