Add sourceIds to KgNodeLabel and :LABELEDGE edges

[123joshuawu]

• Changed writeLabelSources to also add :SOURCE edges to :Label nodes
• Added writeLabelEdgeSources which sets the sourceIds of a :LABELEDGE edge to the intersection set of the setsof sources of the source :Label node and target :Label node
• Add sourceIds to KgNodeLabel
  • The set of sourceIds differs depending on the context
  • KgNodeContext - it is the sourceIds of the corresponding :LABELEDGE edges
  • KgNodeLabelContext - it is the set of sourceIds of the corresponding :Label node

Closes #251

[gordom6]

The synthesized node label<->node label edges should have sources that are the union of the constituent edges connecting any two nodes with those labels.

I've split off this comment into issue #259 and changed the front end. The above part of the comment still needs to be addressed or verified that it was already addressed, in both stores.

[gordom6]

Closed this PR because the follow-ups (#259 and #260) are unrelated.