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.
Remove `_object` ... everything in Python is an object, so putting this in the name is not useful.

Please check the rendered html version for the docstrings you write, in this case you need this to generate an HTML link:
```
psf : `~gammapy.irf.EnergyDependentTablePSF`
```