Combine zcat and mag stages

[griffin-h]

It occurred to me after our discussion today that it would be simpler to avoid mixing aperture and PSF photometry if there were not two separate stages where the user has to specify which one to use. The mag stage is a very simple operation: applying previously calculated zero points and color terms to the supernova magnitude. I can't think of a reason this can't be done at the same time that the zero points and color terms are calculated. In fact it would be easier to do it that way.

This would be a relatively simple change for normal images. It might take some thinking to apply it to difference images as well.

This would also avoid the confusing behavior when the magnitude stage is run with only a subset of the data (e.g., Issues #23 and #66).

[griffin-h]

Standard star images are the only ones where zcat is run but not mag.