Merge edges from different KPs by primary_knowledge_source?

[amykglen]

discussed in today's AHM

right now Expand does not merge edges from different KPs

but there is some duplicated information between KPs, which merging based on primary_knowledge_source may help eliminate

it may not be perfect (e.g., the clinical trials KP may list different primary sources vs. what KG2 lists for its ingested CTKP edges), but it should at least be an improvement! and we could refine the tricky merging over time...