This is a little bit confusing, since we normally think of the flux in a GSObject as being photons, so I'd recommend adding the following to help try to make it a bit clearer (assuming I got it right -- please correct if not):
```
Typically, the SED describes the flux in photons per nanometer of an object with a particular magnitude, possibly normalized with the method `sed.createWithFlux` (see the docstrings in the SED class for details about this and other normalization options).  Then the `flux` attribute of the GSObject should just be the _relative_ flux scaling of the current object compared to that normalization.  This implies (at least) two possible conventions.
1. You can normalize the SED to have unit flux with `sed = sed.createWithFlux(bandpass, 1.0)`.  Then the `flux` of each GSObject would be the actual flux in photons when observed in the given bandpass.
2. You can leave the object flux as 1 (the default for most types when you construct them) and set the flux in the SED with `sed = sed.createWithFlux(bandpass, flux)`.  Then if the object had `flux` attribute different from 1, it would just refer to the factor by which that particular object is brighter than the value given in the normalization command.
```
Also, I'm not wild about the `createWith` names.  I think these might be better as `normalizeFlux` and `normalizeFluxDensity`.  Similar to my comment above about the other `create` method.  It seems overly verbose when all similar methods returna new object.  No need to be explicit about that in the name of the method.