KGX merge overwrites "provided_by" property

[stimon]

Hi,

I'm experimenting with merging different KG-OBO graphs and other source data that have values for the provided_by node property. However, the resulting graph sets the value "Graph" for all nodes. Is there a way to prevent this?

I have been diving both into the documentation and code, but I don't seem to find the source of this problem. As #414 points out, it would be nice to have more descriptive documentation about what can be done through the configuration files :)

Thanks