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.
`gll_iem_v05_ref1.fit` is a 500 MB file.

Can we avoid this in the Sphinx docs build?
I think travis-ci will fail always and often to download it, like it did [here](https://travis-ci.org/gammapy/gammapy/jobs/31345229#L1335)

Can you cut out a small part using ftcopy like I did [here](https://github.com/gammapy/gammapy/tree/master/gammapy/datasets/data/fermi#details)
and put the resulting file (which should be a few MB at most) [here](https://github.com/gammapy/gammapy-extra/tree/master/datasets/vela_region)?

If it's still large (say more than 5 MB), you could just use a cutout of one of the older, lower-resolution diffuse model files?