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.
Adding an assert here for the npred count sum would be better ... it's higher precision.
The test against gtmodel is via the Vela dataset, which unfortunately can't be used for unit tests because it's remote data.