Merge disease-variant records, similar to disease-gene records

[colleenXu]

Main issue: After the discussion in today's biothings meeting, we agreed that these two endpoints, disgenet.genes_related_to_disease and disgenet.variants_related_to_disease, should have similar behavior. genes_related_to_disease has a step to merge records when all columns have the same value except pubmed that variants doesn't.

Also:

• the source field is now expected to be 1 value (not ";" delimited) due to us using different files. so these lines are likely not needed.
• edit documentation to make the newer format clear.

I plan to make a PR with edits for these issues. Then hopefully @kevinxin90 can check that it works and edit it if needed.

[colleenXu]

This issue has been addressed and is done. MyDisease does this now.