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.
It's not necessary here specifically; I did need it in a few early test cases where the spatial binning was not the same, and left it in to keep the example more general... I guess people can figure out themselves whether or not this is required in their case though, so this isn't needed any more.