Needs ._fh handle to stream to be set by child

def _fetch_headers(self):
    """Needs ._fh handle to stream to be set by child"""
    self._header_text, self._n_ref = self._read_top_header()
    self._ref_lengths, self._ref_names = self._read_reference_information()
    self._header = SAMHeader(self._header_text)