Reads the reference names and lengths

def _read_reference_information(self):
    """Reads the reference names and lengths"""
    ref_lengths = {}
    ref_names = []
    for n in range(self._n_ref):
      l_name = struct.unpack('<i',self._fh.read(4))[0]
      name = self._fh.read(l_name).rstrip('\0')
      l_ref = struct.unpack('<i',self._fh.read(4))[0]
      ref_lengths[name] = l_ref
      ref_names.append(name)
    return ref_lengths, ref_names