Download files for SNP markers that have been genetically mapped
by at least on map held by MaizeGDB.

Files:
  MaizeGDB_genetic_SNPs.xref 
     - SNP ids, position in B73 v3 and v4, associated gene models, 
       position in 5 major maps (Cornfed Dent Composite, 
       Cornfed Flint Composite, IBM MaizeSNP50, LHRF Gnp2004, NAM)

  MaizeGDB_genetic_SNPs.fa
    - Sequence for SNP markers that are genetically mapped at
      MaizeGDB.