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.
It's weird that the diffuse model counts are above the true counts, because there are additional sources like Vela in the region, so the true counts should really be above the diffuse model counts.
(I still didn't get this example to run, so I haven't seen the image.)

Add a sentence stating this and make a guess that possibly the explanation is that the diffuse model is too high in this region?