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.
At the moment the .flux method in GammaSpectralCube currently returns an array only with units '1/(cm2 s sr)', and in cases like counts or exposure cube, this isn't correct (we said this would be changed in some future PR, but it would be quite some work) - so I have to just extract values and assign the correct units here (using the .to method would either affect the values or not work when not dimensionally consistent).