Read the header text and number of reference seqs

def _read_top_header(self):
    """Read the header text and number of reference seqs"""
    magic = self._fh.read(4)
    l_text = struct.unpack('<i',self._fh.read(4))[0]
    header_text = self._fh.read(l_text).rstrip('\0')
    n_ref = struct.unpack('<i',self._fh.read(4))[0]
    return header_text, n_ref