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 think the search radius is 5 deg actually.
And the GLAT given above is incorrect.

I'd suggest to just add a link to the README in the gammapy-extra repo to avoid duplicating this info and having to check / adapt it in multiple places.