docs: def convert_ensembl_to_entrez(self, ensembl):
          """Convert Ensembl Id to Entrez Gene Id"""
          if 'ENST' in ensembl:
              pass
          else:
              raise (IndexError)
          # Submit resquest to NCBI eutils/Gene database
          server = "http://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?" + self.options + "&db=gene&term={0}".format(
              ensembl)
          r = requests.get(server, headers={"Content-Type": "text/xml"})
          if not r.ok:
              r.raise_for_status()
              sys.exit()
          # Process Request
          response = r.text
          info = xmltodict.parse(response)
          try:
              geneId = info['eSearchResult']['IdList']['Id']
          except TypeError:
              raise (TypeError)
          return geneId