Change: Add citeAs element with cardinality 0..1 and datatype Reference(Citation) to the Evidence Resource. Short name/description = "Citation for this Evidence"

Rationale: In clinical decision support with calculations based on a precise item of Evidence there is a desire to precisely cite the supporting evidence. Where precision matters make direct citations as precisely as possible. Sharing how to cite information with the target resource will make this easier. The Evidence.citeAs.display value should include the human-readable display for how the Evidence Resource should be cited.

We considered leaving citeAs element limited to EvidenceReport Resources where this is more traditional in publishing. However the multiple use cases where the specific evidence is what is desired would lead to creation of many EvidenceReport Resources containing only 1 Evidence Resource just to use the citeAs element. There is no backward compatibility concern because this is adding a new element.