Create mapping file for uniprot

[susannasiebert]

From #1342:

We also need you to provide us with a tab delimited mapping file with the uniprot accession and the identifier we use to link to you. In the examples you mentioned this would be either the gene symbol "BRAF" or "5". Personally I think BRAF is best. So it might look like.

P15056 BRAF
Q9UM73 ALK

If there is a url to your APIs that can produce a tab delimited list like this we can use that also. We have too many xrefs to process to maintain parsers to different formats various resources use so we ask for an ftp or http(s) link to get or create the file.

We might also want to make use of the following feature:

Note: there is an optional 3rd column with additional information as I described in the xref file if we want to pursue that

[malachig]

How about for the 3rd column we say something like this:

W clinical assertions and X evidence items for Y variants curated from Z published sources.