Source references

[allisonheath]

Not sure the right term for this, but there was this concept for elements of Phenopackets to have (optionally) references to deeper data that could be useful for more fine grained analysis. There is "evidence" currently, but this would likely be a more full fledged URI to source data.

There seems to be two categories at the moment: FHIR profiles/resources and data files. But if others have use cases that would help extrapolate how general this should be and if there are other GA4GH standards that can be the recommended a la #142)

Just a few example use cases:

• The pathology report/slide images for cancer histology. For example, you're doing an automated analysis and see three outliers, this would allow retrieval of the report to determine for example if the diagnosis was borderline or had unusual characteristics.
• A full EHR level treatment FHIR bundle to associated with a cancer protocol.
• Photographs of physical phenotypes

[allisonheath]

May be a dup/closely related to #144