docs: def get_deprecated_gene_ids(filename):
      """ gets a dict of the gene IDs used during in DDD datasets that have been
      deprecated in favour of other gene IDs
      """
      
      deprecated = {}
      with open(filename) as handle:
          for line in handle:
              line = line.strip().split()
              old = line[0]
              new = line[1]
              deprecated[old] = new
      
      return deprecated