Npred cube2

Implementation which I intend to use to produce significance images (currently this is still running on my machine).

The unit tests will need much improvement soon as I just implemented some quick ones. Looking at the outputs with ds9 using the FermiGalacticCenter files seemed reasonable, but obviously not rigorous yet.

Don't know if you were aiming to merge this asap, so did a pull request just in case.
I still can't get APLPy to work here without writing a fits file to disk, which probably isn't desirable.

The error I get is: 

/home/ellis/.local/lib/python2.7/site-packages/APLpy-0.9.12.dev620-py2.7.egg/aplpy/wcs_util.pyc in wcs_pix2sky(self, x, y, origin)
    157                 return yw, xw
    158             else:
--> 159                 return AstropyWCS.wcs_pix2sky(self, x, y, origin)
    160         else:
    161             coords = []

AttributeError: type object 'WCS' has no attribute 'wcs_pix2sky'

In the meantime, I will stick with matplotlib with a loop or list of some kind, like you said