I'd like ``ccdproc`` and ``photutils`` to work well with each other, of course, but I think we have to assume there will be users who want or need to use one but not the other.

``CCDData`` currently does not store a gain; the only things that distinguish it from plain ``NDData`` right now are that the uncertainty is a ``StdDevUncertainty`` and it requires that a unit be set. The gain isn't even required to be in the metadata (and is currently ignored even if it is in the metadata, actually); when we need it, there is a function argument for providing it.