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.
It's error-prone and inconvenient that the caller has to normalise the kernel.
Please add a `normalize` option to the [TablePSF.kernel](https://gammapy.readthedocs.org/en/latest/api/gammapy.irf.TablePSF.html#gammapy.irf.TablePSF.kernel) method that is `True` by default (and add a test to show that it works).
Then remove this line here.