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.
I changed the name as you suggest.

We didn't make this a class method as you planned to make big changes to GammaSpectralCube (i.e. have separate classes for Spectral cubes, counts cubes, exposure cubes, etc) and it isn't currently clear how best this will fit in, so it seemed sensible to just leave it is a function for now.