code: def _averageFromHeader(self, header, keyword):
          
          _list = ''
          for _kw in keyword.split(','):
              if _kw in header:
                  _list = _list + ',' + str(header[_kw])
              else:
                  return None
          return self._averageFromList(_list)
docs: Averages out values taken from header. The keywords where
              to read values from are passed as a comma-separated list.