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.
If you have 261 true counts and 352 predicted counts from `gtmodel`, obviously the `gtmodel` npred image is incorrect.

I guess the reason is that you don't have enough energy bins in the `npred` computation?
Can't you increase those by a factor of a few to get a correct npred image?