Converts a basepol into a tuple of (ant1, ant2, pol).

def bp_to_aap (bp):
    """Converts a basepol into a tuple of (ant1, ant2, pol)."""

    ap1, ap2 = bp
    if ap1 < 0:
        raise ValueError ('first antpol %d is negative' % ap1)
    if ap2 < 0:
        raise ValueError ('second antpol %d is negative' % ap2)

    pol = _fpol_to_pol[((ap1 & 0x7) << 4) + (ap2 & 0x7)]
    if pol == 0xFF:
        raise ValueError ('no CASA polarization code for pairing '
                          '%c-%c' % (fpol_names[ap1 & 0x7],
                                     fpol_names[ap2 & 0x7]))

    return ap1 >> 3, ap2 >> 3, pol