bandpass definitions, demonstrations, etc.

[rmandelb]

As discussed in #923, there are a few issues related to our bandpass definitions:

1. The zeropoints are bandpass zeropoints (mag of object producing 1 count/sec/cm^2 through that bandpass), while many astronomers are used to instrumental bandpass zeropoints (mag of object producing 1 count/sec in that instrument for that bandpass). Overloading terminology can lead to confusion. We should think about whether we want any changes.

2. Quoting @jmeyers314 : "We could definitely use a more extensive demo for setting SED fluxes/magnitudes/flux-densities, Bandpass zeropoints, using the drawImage area and exptime kwargs, and using the RealGalaxy and ChromaticRealGalaxy area_norm kwarg." Some of the demo12 documentation may even be slightly wrong after our latest updates to how chromatic images are normalized.