Add option to provide clique leaders in addition to the combined clique

[gaurav]

When we have multiple conflations in effect (e.g. NCATSTranslator/Babel#440 (comment)), it will be useful for people to be able to see what individual cliques are being conflated. At a minimum this information can be provided as either:

{ "conflated_cliques": [ {"id": "CHEBI:1234", "type": "biolink:ChemicalEntity", "identifiers": ["PUBCHEM:123"]} ] }

Or:

{ "conflated_cliques": { "biolink:ChemicalEntity": ["CHEBI:1234", "UMLS:1233"], "biolink:Gene": ["NCBIGene:ABCD"]}}

The extreme version to this would actually provide per-clique identifier information (i.e. provide the same output as if each clique had been queried separately). I'm not familiar enough with NodeNorm to be sure whether that will be more or less work than just providing a list of cliques, but I suspect it might be the easiest way to accomplish this.