Register precomputed pivot wavelengths.

def register_pivot_wavelength(self, telescope, band, wlen):
        """Register precomputed pivot wavelengths."""
        if (telescope, band) in self._pivot_wavelengths:
            raise AlreadyDefinedError('pivot wavelength for %s/%s already '
                                      'defined', telescope, band)
        self._note(telescope, band)
        self._pivot_wavelengths[telescope,band] = wlen
        return self