Move some functions "upstream"

[cdeil]

@crawfordsm @mwcraig I just saw the ccdproc 0.2 release announcement ... congratulations!

Looking through the list of functions in the ccdproc API docs I think some should be moved "upstream", specifically:

• sigma_func could be moved to astropy.stats. (I've seen this factor 1.48 also show up in the photutils docs, so it's definitely something that should be moved to the Astropy core)
• trim_image and rebin functionality be available via the imageutils module soon, which is planned to become available in the core as astropy.image. You might still need a wrapper that can deal with CCDData objects, but we should keep in contact to make sure we don't duplicate basic image utils functionality.

[crawfordsm]

Thanks @cdeil !

Yup, I think this was some of our motivation for keeping track of photutils and other packages is to avoid unnecessary duplication.

One thing that would be probably useful is to make many of these tasks operate on a generic NDData object as well as a numpy array instead of just CCDData. In that sense, the functions could be useful for a number of different things. It might be a good time to have a review of the many different on-going projects to see what overlaps we have and what should be moved upstream.

I'll make a pull request for sigma_func to astropy.stats as soon as I can.

Let us know if you'd like us to contribute the other functions to imageutils. If the functions are operating on a NDData object, then they should be useful to ccdproc.

[crawfordsm]

trim_image is mentioned in #175 and also rebin and sigma_func are now in astropy.