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.
So, re-running gtmodel with 100 energy bins in the npred computation (instead of 3) gives 285 counts, which seems much more reasonable...

I guess this improved gtmodel result should also be included in gammapy-extra (?) as it would be nice to include a better gtmodel example in the tutorial... should I replace the existing gtmodel npred cube in gammapy-extra, or provide this as an extra one?