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.
Did you update these numbers after removing the isotropic diffuse model?

And if you forgot to run gtmktime before, please update them again.

Probably you'll find npred below the actual counts and can remove the following sentence.