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.
This looks overly complicated.
You call the [solid_angle](https://gammapy.readthedocs.org/en/latest/_modules/gammapy/image/utils.html#solid_angle) function which returns a quantity, call that an array, then access `.value` to get at the actual array and again make a quantity.

Looking at the implementation of the `solid_angle` function (please add this to the doctring there), it does return a Quantity in units `steradian`, so you can simply return that?