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 have changed the docstring to say it returns a Quantity so at least it's consistent - I didn't write this particular function, so I don't really want to make changes to it (I imagine switching to returning an array could lead to needing to make changes elsewhere as the function is used in a number of places in gammapy, and this could end up being quite a bit of work...)