DocumentReference attachment/url check

[gotdan]

Currently, the us-core-6 attachment data/url check for DocumentReference is skipped since Cumulus strips that element. This makes sense once the data has been loaded, but it's an important check if we plan to process the notes coming from bulk data using NLP. Should we have a flag to enable this, or alternatively, add a "NLP ready" version of the metric that inherits from the US Core metric and adds this check?

[mikix]

We talked about this off-github and we agree there's probably utility in a docref attachment metric talking about content types and all that jazz - but it would need to run in a mode that operated on the original ndjson, not the post-ETL ndjson

[mikix]

Also see #23

[mikix]

One piece of this puzzle that would allow analyzing url/data fields en masse (not just local data mode) is smart-on-fhir/cumulus-etl#361