code: def populateFromFile(self, dataUrl):
          
          self._dataUrl = dataUrl
          fastaFile = self.getFastaFile()
          for referenceName in fastaFile.references:
              reference = HtslibReference(self, referenceName)
              # TODO break this up into chunks and calculate the MD5
              # in bits (say, 64K chunks?)
              bases = fastaFile.fetch(referenceName)
              md5checksum = hashlib.md5(bases).hexdigest()
              reference.setMd5checksum(md5checksum)
              reference.setLength(len(bases))
              self.addReference(reference)
docs: Populates the instance variables of this ReferencSet from the
          data URL.