validation issues with MDC CodeSystem

[jimsteel]

I'm seeing some validation issues with the MDC CodeSystem in fhir.tx.support.r4. Notably:

• uses CodeSystem.concept.description instead of CodeSystem.concept.definition
• has a bunch of acronym designations with "value" : ""

I can submit a pull request with fixes, but I thought I'd check first to see what its provenance is, in case these problems need to be reported upstream somewhere.

[grahamegrieve]

@mfaughn and @stefan-karl comments please

[stefan-karl]

The MDC CodeSystem resource is generated from RTMMS (https://rtmms.nist.gov/), maintained by @mfaughn. Fixing should be done in RTMMS implementation, otherwise the same issues will show up again in the next update.

[mfaughn]

Submitted PR that fixes "description" to "definition" and elides all acronym designations with blank values. Also added a new property, FWIW.