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.
Can you write the `ENERGIES` extension here as well, please?
```
$ ftlist ./gammapy/datasets/data/fermi/gll_iem_v02_cutout.fits H

        Name               Type       Dimensions
        ----               ----       ----------
HDU 1   Primary Array      Image      Real4(61x21x30)               
HDU 2   ENERGIES           BinTable     1 cols x 30 rows            
```

`astropy.table.Table` is not properly integrated with `astropy.io.fits` yet, so for now I think you have to create a `BinTableHDU` object yourself (see example [here](https://github.com/gammapy/gammapy/blob/7a0b42e1b2d0d1d2079493d6567516c57fd9fe50/gammapy/irf/psf_table.py#L467).