raise ValueError when invalid model provided to relativeairmass

[wholmgren]

The atmosphere.relativeairmass function currently assumes the 'kastenyoung1989' model if the specified model is invalid. The Python convention for this is to raise a ValueError, and I think that's the right thing to do here too. Since 0.3 is going to break everyone's code anyways, this is a good opportunity to fix this small problem.

    if ...

    elif 'gueymard1993' == model:
        AM = ( 1.0 / (np.cos(zenith_rad) +
            0.00176759*(z)*((94.37515 - z) ** - 1.21563)) )
    else:
        pvl_logger.warning("%s is not a valid model type for relative airmass. The 'kastenyoung1989' model was used.",
                           model)
        AM = ( 1.0 / (np.cos(zenith_rad) +
            0.50572*(((6.07995 + (90 - z)) ** - 1.6364))) )

PRs welcome.