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.
Actually a method `to_fits` that returns a `HDUList` object would be more flexible, because then the user can decide if he wants to save it to file or not. (see example [here](https://gammapy.readthedocs.org/en/latest/_modules/gammapy/irf/psf_table.html#EnergyDependentTablePSF.to_fits).