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.
Reprojecting counts is always problematic because you get non-integer output and Poisson stats are incorrect ... It should not be necessary here, I.e. Counts and exposure should have the same spatial binning, no?