Make xrefs between Ensembl IDs and Uniprot IDs in STRING ingest

[justaddcoffee]

To partially address #235

This PR:

• downloads mapping between Ensembl IDs and Uniprot IDs
• makes an xref edge between equivalent Ensembl IDs and Uniprot IDs in STRING edges.tsv
• makes a node for the Uniprot ID in STRING nodes.tsv
• also adds a bunch new unit tests to increase test coverage

[justaddcoffee]

@deepakunni3 please have a look - I think these last few commits give us what we want.

Note that we have both alias (which contains NCBIGenes ids for genes) and xrefs (which contain UniprotKB ids for proteins) - this is what we want right?

id	name	category	description	alias	xrefs	provided_by
ENSEMBL:ENSG00000004059	ARF5	biolink:Gene	ADP ribosylation factor 5	NCBIGene:381		STRING
ENSEMBL:ENSP00000000233		biolink:Protein			UniprotKB:P84085	STRING