Some disgenet data lacking disgenet.xrefs

[colleenXu]

(EDIT: found while working on biothings/biothings_explorer#900)

Currently, BTE uses the field disgenet.xrefs.umls to retrieve disgenet data because it's the single-namespace disease-ID field that covers the most data: 22574 / 27431 documents with disgenet.

But that's still a good chunk of data that's not accessible: 4857 documents. Today I dug a little more and realized they don't have the parent field disgenet.xrefs either. Instead, the disease ID seems to be in _id only, but with no prefix to say what the ID-namespace is.

I think this may be worth digging into more, and perhaps:

• adding the disgenet.xrefs field to all documents with disgenet. Probably would include at least the disgenet.xrefs.umls and disgenet.xrefs.disease_name fields (I think disgenet data is based on disease UMLS IDs?)
• adding the ID-prefix to _id