Annotate each key with the source of the data

[stuppie]

There should to be a way to determine where exactly each field within each document comes from. This is necessary to be able to accurately use the data. This can lead to confusion. For instance, genes are indexed by their Entrez IDs, but the genomic positions come from Ensembl. I don't see anything in the documentation ?

Currently, there is no way to determine the source of a particular field without looking through the source code. Even if the source of each field was described in the documentation, this is not ideal because certain fields may come from multiple different sources depending on the document you are looking at. For that reason, it would be more helpful if each document contained metadata describing where each field came from. This can, of course be optional.