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.
Won't changing the sign of `CDELT1` parameter for the WCS in the header here lead to an incorrect WCS?
Remove this line?

I know the Fermi diffuse model has positive `CDELT1` and astro images and the Fermi count images have negative `CDELT1`. But that's OK ... the `reproject` method should take care of this when you project the diffuse model to match your data cube grid.