Add clarification / guidance to definition of Coding

[mbrush]

This issue pertains to the proposal for updating the Coding definition in PR #17 .

As presently written, the definition of Coding and its properties are a bit too minimal IMO, and would benefit from a little implementation guidance. PR #17 aims to provide this - with inspiration taken from FHIR Coding spec (link), and from Bob's initial stab at a data type spec for GA4GH (link).

In short, the proposed enhancements give more guidance around acceptable and preferred form/syntax for specifying a code, and a code system. This guidance is still quite permissive (e.g. recommends but does not require the system be entered as a uri/url, and that the code to be represented as a CURIE where possible). As the model matures we can apply tighter constraints on certain fields/values if we choose to. We can also define clear conventions around the use of CURIEs here and in other elements of the model. But for now I think the minimal recommendations provide din PR #17 are useful and sufficient.

An example of how I envision a well-formed Coding object looking is below (it holds a code from the Contributor Role Ontology as the value of a Contribution.activity attribute).:

"activity”: {
   “code”: “CRO:0000001”      # note this is a CURIE
   “label”: “author role”
   “system”: “http://purl.obolibrary.org/obo/cro.owl”     # note this is a URL - specifically the url that the CURIE prefix expands to
   “systemVersion”: 1.0,
}

But as noted, the draft in PR #17 allows for cases where the code is not a CURIE (can be anything, e.g. AFR), and the system does not have a proper uri/url (e.g. "Gnomad-Defined Race Codes").

[larrybabb]

I'm fine with this upgrade as it still allows the implementers the ability to deviate if/when their codes are not representable as CURIEs (which will definitely happen).

[larrybabb]

@mbrush I'm not sure if this needs further discussion/consideration. I believe you and I agreed that this is a fine recommendation. Since you updated the Coding.code description to reflect this preference. I believe this is all set. Please reopen if desired and state what is left to do if anything.