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.

[rmjarvis]

Is there still thought to be any confusion with respect to our zeropoint definitions? Seems like maybe this issue could be closed as no longer relevant.

[rmandelb]

There have been enough changes to the code since then and extensive usage of this functionality, and I don't get the sense that this issue is significant. Let's close it and if somebody has a more specific example of current usage where there's a problem, they can open a new and more relevant issue.