Add source column to gene-to-phenotype association table - not just "evidence"

[matentzn]

https://monarchinitiative.org/HP:0000822?associations=biolink:GeneToPhenotypicFeatureAssociation

We currently have the evidence column, but I think it makes sense to more clearly indicate the source of the association right when you see it. As a researcher I want to at least now if the association is experimentally validated at a single glance, not just deduced from say existing g2d's (d!).

[rays22]

For the sake of checking accuracy and reproducibility, it is of paramount importance to clearly distinguish empirical evidence (observed gene-phenotypic feature associations) from assumptions (inferred, potential gene-phenotypic feature associations). In order to allow that, you need to indicate the source(s) for each gene-phenotypic feature association, so that the supporting evidence can be traced to the source and checked. Ideally, the source should be a study published in a peer reviewed publication, but it could also be a reference to another database, or well-documented evidence codes for unpublished computational inferences. You can find nice examples of how to provide the sources for gene-phenotype associations in tabulated formats in model organism databases.